build: Add sphinx/breathe support for generating documentation

This is adds basic configuration files for doxygen and for breathe,
which is a doxygen-to-sphinx bridge that can document C symbols.

Breathe is configured with default project 'weston' and implicitly adds
:members: and :undoc-members: to breathe configuration options.
This allows a shorter way to call breathe directives without the need
specify the project and also to display implicitly all the members,
documented or not.

A 'docs' run_target to force the docs to be re-built has been added.
Initially (the first time the build system is ran) the documentation
will automatically be built, but later re-builds will require the use of
the 'docs' target. This avoid further delays in building weston but in
the same time allows the possiblity to update/improve the documentation
bits to those who want that.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>