Start on rustdoc crawler (#12942)

This PR adds a first pass at a rustdoc crawler.

We'll be using this to get information about a crate from the rustdoc
artifacts for use in the Assistant.

Release Notes:

- N/A

---------

Co-authored-by: Richard <richard@zed.dev>

crates/html_to_markdown/src/html_to_markdown.rs

@@ -58,7 +58,7 @@ pub fn convert_rustdoc_to_markdown(html: impl Read) -> Result<(String, Vec<Rustd
     let items = item_collector
         .borrow()
         .items
-        .values()
+        .iter()
         .cloned()
         .collect::<Vec<_>>();