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/rustdoc/src/rustdoc.rs

@@ -1 +1,6 @@
 pub mod crawler;
+mod item;
+mod to_markdown;
+
+pub use crate::item::*;
+pub use crate::to_markdown::convert_rustdoc_to_markdown;