Support `setext` style md headings in menu item parsing

Parsing md headers from the readme + api reference into a packages navigation items currently does not support [`setext`](https://github.github.com/gfm/#setext-headings) style headings. This causes these items to not be displayed in the left nav of a package page.

Locate the menu item parsing entry point that reads README and API reference Markdown, then reproduce the issue with a setext-style heading. Done means headings using this syntax produce package navigation items that appear in the page's left nav.

Written by the indexing model from the issue text.