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/docs_preprocessor/src/templates.rs

@@ -0,0 +1,25 @@
+use crate::PreprocessorContext;
+use regex::Regex;
+use std::collections::HashMap;
+
+mod action;
+mod keybinding;
+
+pub use action::*;
+pub use keybinding::*;
+
+pub trait Template {
+    fn key(&self) -> &'static str;
+    fn regex(&self) -> Regex;
+    fn parse_args(&self, args: &str) -> HashMap<String, String>;
+    fn render(&self, context: &PreprocessorContext, args: &HashMap<String, String>) -> String;
+
+    fn process(&self, context: &PreprocessorContext, content: &str) -> String {
+        self.regex()
+            .replace_all(content, |caps: &regex::Captures| {
+                let args = self.parse_args(&caps[1]);
+                self.render(context, &args)
+            })
+            .into_owned()
+    }
+}