Many updates to reftest documentation

Hopefully this is the last big batch of changes.

- Changes to reftests.md:
    - Folded the 'reftest-main-tutorial.md' doc into this one. It wasn't really a tutorial, but an example -- and a good one of a pure HTML reftest. 
    - Add a CSS example in addition to the new HTML example and the previous SVG example
    - Added 'When to Write' and 'How to Run' sections, with more emphasis on
    automation
    - Added info about class="reftest-wait"
    - Removed info about the Mozilla manifest. It is deprecated and AFAIK, not 
    used anywhere in W3C-land
    - Minor edits and formatting fixes and removal of redundant info 
      
- Minor edits and formatting fixes in test-format-guidelines, test-style-guidelines, and test-templates, including rearranging the TOC to appear beneath the page title