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

Documentation/gpu/index.rst

@@ -16,6 +16,7 @@ Linux GPU Driver Developer's Guide
    vga-switcheroo
    vgaarbiter
    todo
+   rfc/index
 
 .. only::  subproject and html
 

Documentation/gpu/rfc/index.rst

+===============
+GPU RFC Section
+===============
+
+For complex work, especially new uapi, it is often good to nail the high level
+design issues before getting lost in the code details. This section is meant to
+host such documentation:
+
+* Each RFC should be a section in this file, explaining the goal and main design
+  considerations. Especially for uapi make sure you Cc: all relevant project
+  mailing lists and involved people outside of dri-devel.
+
+* For uapi structures add a file to this directory with and then pull the
+  kerneldoc in like with real uapi headers.
+
+* Once the code has landed move all the documentation to the right places in
+  the main core, helper or driver sections.