assistant: Add `/rustdoc` slash command (#12453)

This PR adds a `/rustdoc` slash command for retrieving and inserting
rustdoc docs into the Assistant.

Right now the command accepts the crate name as an argument and will
return the top-level docs from `docs.rs`.

Release Notes:

- N/A
08881828ceMarshall Bowers committed on 5/29/2024, 10:14:29 PM· committed by GitHubparentdd328ef
8 files changedLine totals unavailable