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/action.rs

@@ -0,0 +1,50 @@
+use crate::PreprocessorContext;
+use regex::Regex;
+use std::collections::HashMap;
+
+use super::Template;
+
+pub struct ActionTemplate;
+
+impl ActionTemplate {
+    pub fn new() -> Self {
+        ActionTemplate
+    }
+}
+
+impl Template for ActionTemplate {
+    fn key(&self) -> &'static str {
+        "action"
+    }
+
+    fn regex(&self) -> Regex {
+        Regex::new(&format!(r"\{{#{}(.*?)\}}", self.key())).unwrap()
+    }
+
+    fn parse_args(&self, args: &str) -> HashMap<String, String> {
+        let mut map = HashMap::new();
+        map.insert("name".to_string(), args.trim().to_string());
+        map
+    }
+
+    fn render(&self, _context: &PreprocessorContext, args: &HashMap<String, String>) -> String {
+        let name = args.get("name").map(String::as_str).unwrap_or_default();
+
+        let formatted_name = name
+            .chars()
+            .enumerate()
+            .map(|(i, c)| {
+                if i > 0 && c.is_uppercase() {
+                    format!(" {}", c.to_lowercase())
+                } else {
+                    c.to_string()
+                }
+            })
+            .collect::<String>()
+            .trim()
+            .to_string()
+            .replace("::", ":");
+
+        format!("<code class=\"hljs\">{}</code>", formatted_name)
+    }
+}

crates/docs_preprocessor/src/templates/keybinding.rs

@@ -0,0 +1,36 @@
+use crate::PreprocessorContext;
+use regex::Regex;
+use std::collections::HashMap;
+
+use super::Template;
+
+pub struct KeybindingTemplate;
+
+impl KeybindingTemplate {
+    pub fn new() -> Self {
+        KeybindingTemplate
+    }
+}
+
+impl Template for KeybindingTemplate {
+    fn key(&self) -> &'static str {
+        "kb"
+    }
+
+    fn regex(&self) -> Regex {
+        Regex::new(&format!(r"\{{#{}(.*?)\}}", self.key())).unwrap()
+    }
+
+    fn parse_args(&self, args: &str) -> HashMap<String, String> {
+        let mut map = HashMap::new();
+        map.insert("action".to_string(), args.trim().to_string());
+        map
+    }
+
+    fn render(&self, context: &PreprocessorContext, args: &HashMap<String, String>) -> String {
+        let action = args.get("action").map(String::as_str).unwrap_or("");
+        let macos_binding = context.find_binding("macos", action).unwrap_or_default();
+        let linux_binding = context.find_binding("linux", action).unwrap_or_default();
+        format!("<kbd class=\"keybinding\">{macos_binding}|{linux_binding}</kbd>")
+    }
+}