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/html_to_markdown/Cargo.toml

@@ -14,10 +14,8 @@ 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