Updates & additions to reftest documentation

Reviewed and made a few changes to the original version of the reftest doc migrated from the CSSWG wiki: 
- Changed the example reftest to be one that uses red/green
- Migrated the test format guidelines and self describing test docs
- Fixed a bunch of formatting issues
- Added the mention of reftests being self-describing tests back - they can be both, but changed the language in the selftest.md doc to reflect that they are sometimes automatable if they're reftests.
- Removed the mention of 'unreftestable tests' from this doc - IMO, this is not the right place for this info - better trackable through Shepherd. 

Note: I had to use plain HTML instead of MD for the Requirements Flags tabe in the test format guidelines doc - it _would not render_ when built with Jekyll.  Ditto for the anchor links from the TOC in that doc.

I think we should discuss how much of this is CSS-specific info and how to present that info here (so as not to put extra requirements for tests for other specs)