docs: add some missing stuff to sourcetree.rst

I alphabetised some lists, but did not attempt to fix the inconsistent
formatting.

v2: added more info
v3: rework for the new format

Signed-off-by: Miklós Máté <mtmkls@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5367>
1 file changed