Switch the documentation to sphinx

This big patch converts the documentation build system to sphinx
(http://sphinx.pocoo.org/

). Since that uses reStructuredText sources
too, there is no change (yet) in the documents themselves, just in the
build system.

As before, the docs are pre built by the maintainer, and the end-user
doesn't need sphinx or other rst tools to build the docs. Note that we
are not distributing PDFs, so building that will require the tools.

The docs will be stored under doc/html and the build system also need an
extra directory doc/build. These are considered (by automake)
maintainer-related objects and are removed at maintainer-clean time.

The patch also fixes some small issues: add a docpng variable, add
doc/api (also generated by maintainer) in maintainer-clean-local, etc.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>