Documentation/gpu: VM_BIND locking document

Add the first version of the VM_BIND locking document which is
intended to be part of the xe driver upstreaming agreement.

The document describes and discuss the locking used during exec-
functions, evicton and for userptr gpu-vmas. Intention is to be using the
same nomenclature as the drm-vm-bind-async.rst.

v2:
- s/gvm/gpu_vm/g (Rodrigo Vivi)
- Clarify the userptr seqlock with a pointer to mm/mmu_notifier.c
(Rodrigo Vivi)
- Adjust commit message accordingly.
- Add SPDX license header.

v3:
- Large update to align with the drm_gpuvm manager locking
- Add "Efficient userptr gpu_vma exec function iteration" section
- Add "Locking at bind- and unbind time" section.

v4:
- Fix tabs vs space errors by untabifying (Rodrigo Vivi)
- Minor style fixes and typos (Rodrigo Vivi)
- Clarify situations where stale GPU mappings are occurring and how
access through these mappings are blocked. (Rodrigo Vivi)
- Insert into the toctree in implementation_guidelines.rst

v5:
- Add a section about recoverable page-faults.
- Use local references to other documentation where possible
(Bagas Sanjaya)
- General documentation fixes and typos (Danilo Krummrich and
Boris Brezillon)
- Improve the documentation around locks that need to be grabbed from the
dm-fence critical section (Boris Brezillon)
- Add more references to the DRM GPUVM helpers (Danilo Krummrich and
Boriz Brezillon)
- Update the rfc/xe.rst document.

v6:
- Rework wording to improve readability (Boris Brezillon, Rodrigo Vivi,
Bagas Sanjaya)
- Various minor fixes across the document (Boris Brezillon)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Acked-by: John Hubbard <jhubbard@nvidia.com> # Documentation/core-api/pin_user_pages.rst changes
Link: https://patchwork.freedesktop.org/patch/msgid/20231129090637.2629-1-thomas.hellstrom@linux.intel.com