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/rustdoc.rs

@@ -1,6 +1,8 @@
 pub mod crawler;
 mod item;
+mod store;
 mod to_markdown;
 
 pub use crate::item::*;
+pub use crate::store::*;
 pub use crate::to_markdown::convert_rustdoc_to_markdown;