docs: move gallium specific docs into gallium folder

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5706>

docs/gallium/postprocess.rst

@@ -0,0 +1,33 @@
+Gallium Post-processing
+=======================
+
+The Gallium drivers support user-defined image post-processing. At the
+end of drawing a frame a post-processing filter can be applied to the
+rendered image. Example filters include morphological antialiasing and
+cell shading.
+
+The filters can be toggled per-app via driconf, or per-session via the
+corresponding environment variables.
+
+Multiple filters can be used together.
+
+PP environment variables
+------------------------
+
+-  PP_DEBUG - If defined debug information will be printed to stderr.
+
+Current filters
+---------------
+
+-  pp_nored, pp_nogreen, pp_noblue - set to 1 to remove the
+   corresponding color channel. These are basic filters for easy testing
+   of the PP queue.
+-  pp_jimenezmlaa, pp_jimenezmlaa_color - `Jimenez's
+   MLAA <https://www.iryokufx.com/mlaa/>`__ is a morphological
+   antialiasing filter. The two versions use depth and color data,
+   respectively. Which works better depends on the app - depth will not
+   blur text, but it will miss transparent textures for example. Set to
+   a number from 2 to 32, roughly corresponding to quality. Numbers
+   higher than 8 see minimizing gains.
+-  pp_celshade - set to 1 to enable cell shading (a more complex color
+   filter).