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/book.toml

@@ -22,3 +22,12 @@ enable = false
 "/python.html" = "/docs/languages/python.html"
 "/adding-new-languages.html" = "/docs/extensions/languages.html"
 "/language-model-integration.html" = "/docs/assistant/assistant.html"
+"/assistant.html" = "/docs/assistant/assistant.html"
+
+# Our custom preprocessor for expanding commands like `{#kb action::ActionName}`,
+# and other docs-related functions.
+#
+# Comment the below section out if you need to bypass the preprocessor for some reason.
+[preprocessor.zed_docs_preprocessor]
+command = "cargo run --manifest-path=../crates/docs_preprocessor/Cargo.toml --"
+renderer = ["html"]