Add docs_preprocessor crate to support Zed Docs (#16700)

This PR adds a mdbook preprocessor for supporting Zed's docs.

This initial version adds the following custom commands:

**Keybinding** 

`{#kb prefix::action_name}` (e.g. `{#kb zed::OpenSettings}`)

Outputs a keybinding template like `<kbd
class="keybinding">{macos_keybinding}|{linux_keybinding}</kbd>`. This
template is processed on the client side through `mdbook` to show the
correct keybinding for the user's platform.

**Action** 

`{#action prefix::action_name}` (e.g. `{#action zed::OpenSettings}`)

For now, simply outputs the action name in a readable manner. (e.g.
zed::OpenSettings -> zed: open settings)

In the future we'll add additional modes for this template, like create
a standard way to render `{action} ({keybinding})`.

## Example Usage

```
To open the assistant panel, toggle the right dock by using the {#action workspace::ToggleRightDock} action in the command palette or by using the
{#kb workspace::ToggleRightDock} shortcut.
```

Release Notes:

- N/A

docs/src/getting-started.md

@@ -42,7 +42,7 @@ If this script is insufficient for your use case or you run into problems runnin
 
 The Command Palette is the main way to access functionality in Zed, and its keybinding is the first one you should make yourself familiar with.
 
-To open the Command Palette, use <kbd>cmd-shift-p|ctrl-shift-p</kbd>.
+To open the Command Palette, use {#kb command_palette::Toggle}.
 
 The Command Palette allows you to access pretty much any functionality that's available in Zed.
 
@@ -54,11 +54,11 @@ Any time you see instructions that include commands of the form `zed: ...` or `e
 
 ## Configure Zed
 
-Use <kbd>cmd-,|ctrl-,</kbd> to open your custom settings to set things like fonts, formatting settings, per-language settings, and more.
+Use {#kb zed::OpenSettings} to open your custom settings to set things like fonts, formatting settings, per-language settings, and more.
 
 On macOS, you can access the default configuration using the `Zed > Settings > Open Default Settings` menu item. See [Configuring Zed](./configuring-zed.md) for all available settings.
 
-On Linux, you can access the default configuration via the Command Palette. Open it with <kbd>ctrl-shift-p</kbd> and type in `zed: open default settings` and then hit return.
+On Linux, you can access the default configuration via the Command Palette. Open it with {#kb zed::OpenDefaultSettings} and type in `zed: open default settings` and then hit return.
 
 ## Set up your key bindings