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/Cargo.toml

@@ -39,6 +39,7 @@ parking_lot.workspace = true
 project.workspace = true
 regex.workspace = true
 rope.workspace = true
+rustdoc_to_markdown.workspace = true
 schemars.workspace = true
 search.workspace = true
 semantic_index.workspace = true