Add delta overview page

docs/packaging/deltas.mdx

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+# Delta Updates
+<AppliesTo all />
+Delta updates provide speedy updates for even the biggest of applications.
+
+By default, as long as there is a previous release in your `--outputDir` / `-o` directory, Velopack will automatically generate a delta when creating a new release.
+
+:::tip
+If you don't have the latest local release, eg. you're running in CI, you can use the `vpk download` command to automatically fetch it so delta's can be created.
+:::
+
+Delta packages use [Zstandard](https://github.com/facebook/zstd) to create binary patches of individual files in your package. Due to limitations of Zstandard, no single file can be bigger than 2 GB.
+
+## Optimising for size
+The default parameters picked for patch generation are to provide a good balance of size and speed. 
+If you would like to optimize for size, you can do this with the `--delta BestSize` argument.
+YMMV, for some appliations this may give you big size savings, for others it may not. Either way it will be a lot slower, about on-par with the `bsdiff` algorithm.
+
+## How delta's work
+During updates, Velopack will look for possibly more than one delta to download. Imagine you have published the following releases:
+- `1.0.0`
+- `1.0.1`
+- `1.0.2`
+- `1.0.3`
+
+If a user has not opened your app in a while, and has `1.0.0` installed, the next time they open the app, 
+3 delta's will be downloaded: `1.0.1-delta`, `1.0.2-delta`, and `1.0.3-delta`.
+
+The previously downloaded/installed package is stored in `%LocalAppData\{packId}\packages`, so Velopack will then apply those 3 delta's sequentially 
+on top of the local package that exists in that directory. 
+
+Once the delta's have been applied, and the `1.0.3` release has been reconstructed, it will be ready for applying. 
+
+There is a heuristic in Velopack which will attempt to decide whether it is better to download N delta's or download the latest full release instead.
+This is based on a number of factors (eg. size of the full release, sum size of the delta's, total number of delta's between the current & target version).
+
+## GitHub Releases
+There is special support for GitHub Releases in `UpdateManager` which will search through a number of previous GitHub Releases for available delta's. 
+This means any one GitHub release must only have 1 full package and 1 delta update to facilitate proper updating. 
+
+## Disabling delta's
+If for some reason you don't want to deploy delta's, you can disable them with the `--delta none` argument.