drm/doc: Add RFC section

Motivated by the pre-review process for i915 gem/gt features, but
probably useful in general for complex stuff.

v2: Add reminder to not forget userspace projects in the discussion
(Simon, Jason)

v3: Actually put this into a folder, so we have it all (.rst files and
headers for kerneldoc) contained somewhere separate (Jason)

Cc: Simon Ser <contact@emersion.fr>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dave Airlie <airlied@redhat.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Simon Ser <contact@emersion.fr>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210325225859.1684116-1-daniel.vetter@ffwll.ch