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

crates/settings/src/keymap_file.rs

@@ -22,10 +22,34 @@ pub struct KeymapBlock {
     bindings: BTreeMap<String, KeymapAction>,
 }
 
+impl KeymapBlock {
+    pub fn context(&self) -> Option<&str> {
+        self.context.as_deref()
+    }
+
+    pub fn bindings(&self) -> &BTreeMap<String, KeymapAction> {
+        &self.bindings
+    }
+}
+
 #[derive(Debug, Deserialize, Default, Clone)]
 #[serde(transparent)]
 pub struct KeymapAction(Value);
 
+impl ToString for KeymapAction {
+    fn to_string(&self) -> String {
+        match &self.0 {
+            Value::String(s) => s.clone(),
+            Value::Array(arr) => arr
+                .iter()
+                .map(|v| v.to_string())
+                .collect::<Vec<_>>()
+                .join(", "),
+            _ => self.0.to_string(),
+        }
+    }
+}
+
 impl JsonSchema for KeymapAction {
     fn schema_name() -> String {
         "KeymapAction".into()
@@ -135,6 +159,10 @@ impl KeymapFile {
 
         serde_json::to_value(root_schema).unwrap()
     }
+
+    pub fn blocks(&self) -> &[KeymapBlock] {
+        &self.0
+    }
 }
 
 fn no_action() -> Box<dyn gpui::Action> {