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

crates/assistant/src/slash_command.rs

@@ -20,6 +20,7 @@ pub mod active_command;
 pub mod file_command;
 pub mod project_command;
 pub mod prompt_command;
+pub mod rustdoc_command;
 pub mod search_command;
 pub mod tabs_command;