Documentation: add a page on amdgpu debugging

Covers GPU page fault debugging and adds a reference
to umr.

v2: update client ids to include SQC/G
v3: Remove duplicate text
v4: add umr documentation link, fix typo

Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>