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