We have a growing set of developer docs but the index is currently in
order of when stuff was added. Try and make things a bit easier to
find my adding sub indexes and organising into themes.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
Message-Id: <20220315121251.2280317-7-alex.bennee@linaro.org>