doc/user: add a hack to get to the git version

doc/user: add a hack to get to the git version

meson doesn't have configuration_data() in vcs_tag so we can only replace one
string. sphinx cannot include things in-line.

Since we want the git version to be replaced in random places, we need to put
it into rst_prolog in conf.py - but that's where we neet to replace other
things too. Work around this by generating a mini python module that returns
the git version, then call that in conf.py.

Side-bonus: we now have access to the full commit and the abbreviated commit.
Not that anything actually uses this...

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>