Update Proto RPC Style / Fix Links

* Replaced all subheadings within a RPC description with bold text. This
  keeps the table of contents clearer and also lets us use more
  descriptive (fake) subheadings. Suggested by @saivann (thanks!)

* Changed all links in _includes/references.md to use full path names to
  /en/developer-guide so they work in the API reference. (Note: this
  breaks our p:hover CSS styling which matched internal links only.)

* Modified _plugins/autocrossref.rb to give all autoxref'd links
  class="auto-link" so we can use the previous styling with a few
  changes to the stylesheet.