Documentation/gpu: Add an explanation about the DCN pipeline

Documentation/gpu: Add an explanation about the DCN pipeline

In the DCN code, we constantly talk about hardware pipeline, pipeline,
or even just pipes, which is a concept that is not obvious to everyone.
For this reason, this commit expands the DCN overview explanation by
adding a new section that describes what a pipeline is from the DCN
perspective.

Changes since V1:
- Rewrite the first paragraph that describes AMD hardware pipeline.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Cc: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Cc: Hersen Wu <hersenxs.wu@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Simon Ser <contact@emersion.fr>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Mark Yacoub <markyacoub@chromium.org>
Cc: Pierre-Loup <pgriffais@valvesoftware.com>
Cc: Michel Dänzer <michel.daenzer@mailbox.org>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>