Rewrite `doc_markdown` to use `pulldown-cmark`
Rewrite `doc_markdown` to use `pulldown-cmark`

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust