Building texinfo version of the manual only works with Sphinx from hg
tip at present.
Also, there are some minor issues and someone should go through the
manual and read it to make sure there aren't issues.
* added a YouCanHelp directive to replace FIXMEs and encourage contributors
to help out
* moved some bits around between the hacking howto and the codebase documents
* expanded on the stub nature of the theming howto
* tweaked some other text
