doc: move drm-misc committer guidelines

Move drm-misc under the common committer guidelines.

v2: s/drm-intel/drm-misc/ under tooling (Emil)

v3: typos, drm-misc small drivers scope (Sean)

Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>