docs: Convert ReleaseNotes to reST.

This is the last of the "regular" documents to convert to reST, and so
I'm declaring the initial clang reST conversion "done".

However,

- There are some documents in clang/www/ which probably should
  be migrated into clang/docs/, such as www/OpenProjects.html

  The primary thing blocking me from doing this right now is not knowing
  how to set up a redirect so that the old URL's continue to work.

- LibASTMatchersReference.html is not reST. This page is auto-generated
  by clang/docs/tools/dump_ast_matchers.py from the source and has some
  collapse/expand logic that isn't expressible directly with Sphinx, so
  just converting it to reST is not really a good strategy.

  Manuel Klimek and I discussed this and the general agreed-upon
  direction is making that page data-driven so that it, say, pulls in an
  auto-generated blob of JSON which describes the matchers and builds up
  the "matcher reference" part of the page with a small amount of JS.

- There are some rogue .txt files hanging around.

Also, I dropped the little dragon logo at the top because Sphinx was
warning about an external image reference (not sure why, but meh, I
didn't want to fight it). If anything, we would want such a logo
integrated into the site's overall theme, rather than hardcoded here.

llvm-svn: 170994