sphinx: Set the html_theme variable explicitly.

This should let our custom css show on readthedocs.org.
Add a sphinx/rst version of the tornadoweb.org index page. (WIP)
Get the tricky parts of the sphinx conversion working.
Add a max-width for body text to match the appengine site
Merge branch 'branch2.4'
Add favicon to sphinx config
Merge branch 'branch2.4'
Move website/sphinx/ to docs/, remove rest of appengine website dir.
Fix sphinx config in its new home, restore Makefile and custom css.
Fix internal links for pdf output.