doc: add common maintainer guidelines file, start with drm-intel

For starters, add the specific guidelines in their own files, starting
with drm-intel, with the intention of consolidating and pulling in
common guides in the top level file in the long term.

Start with adding detailed maintainer tasks for handling drm-intel
branches.

v2: some tweaks, use dim rebase for rebases

v3: clarify gvt features pull for dinq (Rodrigo)

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>