Documentation/gpu: Reorganize DC documentation

Display core documentation is not well organized, and it is hard to find
information due to the lack of sections. This commit reorganizes the
documentation layout, and it is preparation work for future changes.

Changes since V1:
- Christian: Group amdgpu documentation together.
- Daniel: Drop redundant amdgpu prefix.
- Jani: Create index pages.
- Yann: Mirror display folder in the documentation.

Reviewed-by: Yann Dirson <ydirson@free.fr>
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>