Documentation/gpu: Add documentation about ring buffer

Documentation/gpu: Add documentation about ring buffer

AMDGPU heavily relies on ring buffers to manage its components; as a
result, it has an elaborate mechanism of operation with multiple details
around it. This commit introduces new documentation on ring buffers,
detailing their management and expanding the explanation of Enforce
isolation. Finally, this commit also adds the documentation available in
the amdgpu_ring.c file to it.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Rodrigo Siqueira <siqueira@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>