add docs for rule action templating experimental feature (#632)

Closes #480

Feedback issue for the reference
actualbudget/actual#3606

This is a starting point for a more fleshed out version, adding examples
in the future would be good

docs-sidebar.js

@@ -208,6 +208,7 @@ const sidebars = {
             'experimental/monthly-cleanup',
             'experimental/oauth-auth',
             'experimental/multi-user',
+            'experimental/rule-templating',
           ],
         },
         'getting-started/tips-tricks',

docs/experimental/rule-templating.md

@@ -0,0 +1,100 @@
+# Rule Action Templating
+
+:::warning
+This is an **experimental feature**. That means we’re still working on finishing it. There may be bugs, missing functionality or incomplete documentation, and we may decide to remove the feature in a future release. If you have any feedback, please [open an issue](https://github.com/actualbudget/actual/issues) or post a message in the Discord.
+:::
+:::warning
+All functionality described here may not be available in the latest stable release. Use the `edge` images for the latest implementation.
+:::
+
+Rule action templating allows rules to dynamically set fields based on transaction data via meta programming inside the rule.
+
+Setting the following fields with a rule template is currently supported:
+- notes
+- date
+- amount
+- payee (name)
+- cleared (although no boolean helper functions are currently supported)
+
+Actual uses [handlebars](https://handlebarsjs.com/) under the hood to process the rule templates. You can find more in depth information about how this works in [their guide](https://handlebarsjs.com/guide).
+
+## Using rule action templating
+You can toggle between the normal and template input modes by clicking the icon to the right of the action input box.
+
+![How to enable rule action templating](/img/experimental/rule-templating/enable-rule-templating.png)
+
+When the template input mode is active you can type your template into the input box as below. This example removes the string " 12345" from an imported payee and sets the payee to this new value.
+
+![How to enable rule action templating 2](/img/experimental/rule-templating/enable-rule-templating-2.png)
+
+## Variables
+
+| Variable              | Type    | Notes                                                                                                           |
+|-----------------------|---------|-----------------------------------------------------------------------------------------------------------------|
+| today                 | date    | Today's date                                                                                                    |
+| account               | id      |                                                                                                                 |
+| date                  | date    |                                                                                                                 |
+| payee                 | id      |                                                                                                                 |
+| imported_payee        | string  |                                                                                                                 |
+| notes                 | string  |                                                                                                                 |
+| amount                | number  | This is stored without the decimal place. ie. 152 will be 15200. {{div amount 100}} can be used to convert back |
+| cleared               | boolean |                                                                                                                 |
+| reconciled            | boolean |                                                                                                                 |
+| imported_id           | id      | ID of the transaction provided from an import source (eg. bank sync/QFX)                                        |
+| is_child              | boolean | Flag for children in a split transaction                                                                        |
+| is_parent             | boolean | Flag for the parent of a split transaction                                                                      |
+| parent_id             | id      | Set if is_child = true                                                                                          |
+| schedule              | id      |                                                                                                                 |
+| starting_balance_flag | boolean | Set if the transaction is a starting balance transaction                                                        |
+| transfer_id           | id      |                                                                                                                 |
+
+## Functions
+### Mathematical
+
+| Function | Arguments           | Notes |
+|----------|---------------------|-------|
+| add      | number1, number2... |       |
+| sub      | number1, number2... |       |
+| div      | number1, number2... |       |
+| mul      | number1, number2... |       |
+| mod      | number1, number2... |       |
+| floor    | number              |       |
+| ceil     | number              |       |
+| round    | number              |       |
+| abs      | number              |       |
+| min      | number1, number2... |       |
+| max      | number1, number2... |       |
+| fixed    | number1, number2... |       |
+
+### Text
+
+| Function   | Arguments                   | Notes                                                                                                      |
+|------------|-----------------------------|------------------------------------------------------------------------------------------------------------|
+| regex      | value, regex, replacement   |                                                                                                            |
+| replace    | value, pattern, replacement | Mimics js replace. When pattern is not as /regex/flags it just uses raw value as opposed to {{regex ...    |
+| replaceAll | value, pattern, replacement | Mimics js replaceAll. When pattern is not as /regex/flags it just uses raw value as opposed to {{regex ... |
+| concat     | values...                   | Joins all arguments together                                                                               |
+
+### Date
+
+| Function  | Arguments    | Notes                                                         |
+|-----------|--------------|---------------------------------------------------------------|
+| addDays   | date, number |                                                               |
+| subDays   | date, number |                                                               |
+| addWeeks  | date, number |                                                               |
+| subWeeks  | date, number |                                                               |
+| addMonths | date, number |                                                               |
+| subMonths | date, number |                                                               |
+| addYears  | date, number |                                                               |
+| subYears  | date, number |                                                               |
+| setDay    | date, day    | Overflows are handled, 0 will set to last day of month before |
+| day       | date         | Extract the day from a date                                   |
+| month     | date         | Extract the month from a date                                 |
+| year      | date         | Extract the year from a date                                  |
+| format    | date, format |                                                               |
+
+### Other
+
+| Function | Arguments | Notes                                                   |
+|----------|-----------|---------------------------------------------------------|
+| debug    | any       | Prints the arguments to the browser development console |