Move rustdoc-related code to rustdoc crate (#12945)

This PR moves the rustdoc-related code out of `html_to_markdown` and
into the `rustdoc` crate.

Release Notes:

- N/A

crates/assistant/Cargo.toml

@@ -42,6 +42,7 @@ parking_lot.workspace = true
 project.workspace = true
 regex.workspace = true
 rope.workspace = true
+rustdoc.workspace = true
 schemars.workspace = true
 search.workspace = true
 semantic_index.workspace = true