Rollup merge of rust-lang#41112 - ollie27:rustdoc_pull, r=GuillaumeGomez

rustdoc: Use pulldown-cmark for Markdown HTML rendering

Instead of rendering all of the HTML in rustdoc this relies on
pulldown-cmark's `push_html` to do most of the work. A few iterator
adapters are used to make rustdoc specific modifications to the output.

This also fixes MarkdownHtml and link titles in plain_summary_line.

https://ollie27.github.io/rust_doc_test/ is the docs built with this change and rust-lang#41111.

Part of rust-lang#40912.

cc @GuillaumeGomez

r? @steveklabnik