weston-log: Start adding documentation for the logging/debugging framework

Adds initial grouping for sphinx/breathe for the logging/debugging
framework. We add a few groups: log (public API), internal-log (private API,
not exported) and debug-protocol, specific to the weston
debug protocol.

In latest version of breathe, '\memberof' command is recognized as such.
But it conflicts with '\ingroup' command and can't be used in the same
time (leading to duplicate symbols), so we follow a simple rule: object
tagging with '\ingroup' then use '\memberof' command for the functions
that work on that object.

There's also a caveat here: we have objects that are private (opaque)
but the functions are public. For those cases we resort to using
'internal-log' for the object (class) and 'log' for the functions.

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