Add tag handler for collecting crate items from rustdoc output (#12903)

This PR adds a tag handler for collecting crate items from rustdoc's
HTML output.

This will serve as the foundation for getting more insight into a
crate's contents.

Release Notes:

- N/A

crates/html_to_markdown/Cargo.toml

@@ -14,8 +14,10 @@ path = "src/html_to_markdown.rs"
 [dependencies]
 anyhow.workspace = true
 html5ever.workspace = true
+indexmap.workspace = true
 markup5ever_rcdom.workspace = true
 regex.workspace = true
+strum.workspace = true
 
 [dev-dependencies]
 indoc.workspace = true