docs: merge HACKING.rst contents into CODING_STYLE.rst

docs: merge HACKING.rst contents into CODING_STYLE.rst

The split of information between the two docs is rather arbitary and
unclear. It is simpler for contributors if all the information is in
one file.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>