Fix broken links (#1366)

Author: IanManske

book/environment.md

@@ -234,7 +234,7 @@ _(Important! The environment conversion string -> value happens **after** the en
 
 ## Removing environment variables
 
-You can remove an environment variable only if it was set in the current scope via [`hide-env`](/commands/docs/hide_env.md):
+You can remove an environment variable only if it was set in the current scope via [`hide-env`](/commands/docs/hide-env.md):
 
 ```nu
 > $env.FOO = 'BAR'

book/loading_data.md

@@ -184,7 +184,6 @@ There are other commands you can use to work with strings:
 
 - [`str`](/commands/docs/str.md)
 - [`lines`](/commands/docs/lines.md)
-- [`size`](/commands/docs/size.md)
 
 There is also a set of helper commands we can call if we know the data has a structure that Nu should be able to understand. For example, let's open a Rust lock file:
 

book/overlays.md

@@ -67,7 +67,7 @@ In the following sections, the `>` prompt will be preceded by the name of the la
 
 ## Removing an Overlay
 
-If you don't need the overlay definitions anymore, call [`overlay hide`](/commands/docs/overlay_remove.md):
+If you don't need the overlay definitions anymore, call [`overlay hide`](/commands/docs/overlay_hide.md):
 
 ```nu
 (spam)> overlay hide spam
@@ -94,7 +94,7 @@ foo
 ───┴──────
 ```
 
-The last way to remove an overlay is to call [`overlay hide`](/commands/docs/overlay_remove.md) without an argument which will remove the last active overlay.
+The last way to remove an overlay is to call [`overlay hide`](/commands/docs/overlay_hide.md) without an argument which will remove the last active overlay.
 
 ## Overlays Are Recordable
 

book/table_of_contents.md

@@ -2,14 +2,14 @@
 
 - [Installation](installation.md) - Installing Nushell
 - [Introduction](README.md) - Getting started
-- [Thinking in Nushell](thinking_in_nushell.md) - Thinking in Nushell
+- [Thinking in Nu](thinking_in_nu.md) - Thinking in Nushell
 - [Moving around](moving_around.md) - Moving around in Nushell
 - [Types of data](types_of_data.md) - Types of data in Nushell
 - [Loading data](loading_data.md) - Loading data and using it
 - [Strings](working_with_strings.md) - Strings, escape characters, and string interpolation
 - [Working with lists](working_with_lists.md) - Working with Nu lists
 - [Working with tables](working_with_tables.md) - Working with Nu tables
-- [Pipeline](pipeline.md) - How the pipeline works
+- [Pipelines](pipelines.md) - How the pipeline works
 - [Configuration](configuration.md) - How to configure Nushell
 - [3rd Party Prompts](3rdpartyprompts.md) - How to configure 3rd party prompts
 - [Custom commands](custom_commands.md) - Creating your own commands