assistant: Add MVP for /rustdoc using indexed docs (#12952)

This PR adds an MVP of retrieving docs using the `/rustdoc` command from
an indexed set of docs.

To try this out:

1. Build local docs using `cargo doc`
2. Index the docs for the crate you want to search using `/rustdoc
--index <CRATE_NAME>`
    - Note: This may take a while, depending on the size of the crate
3. Search for docs using `/rustdoc my_crate::path::to::item`
    - You should get completions for the available items

Here are some screenshots of it in action:

<img width="640" alt="Screenshot 2024-06-12 at 6 19 20 PM"
src="https://github.com/zed-industries/zed/assets/1486634/6c49bec9-d084-4dcb-a92c-1b4c557ee9ce">

<img width="636" alt="Screenshot 2024-06-12 at 6 52 56 PM"
src="https://github.com/zed-industries/zed/assets/1486634/636a651c-7d02-48dc-b05c-931f33c49f9c">

Release Notes:

- N/A

crates/rustdoc/src/item.rs

@@ -43,6 +43,13 @@ pub struct RustdocItem {
 }
 
 impl RustdocItem {
+    pub fn display(&self) -> String {
+        let mut path_segments = self.path.clone();
+        path_segments.push(self.name.clone());
+
+        path_segments.join("::")
+    }
+
     pub fn url_path(&self) -> String {
         let name = &self.name;
         let mut path_components = self.path.clone();