docs: add basic docs for d3d12 driver

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7687>

docs/contents.rst

@@ -50,6 +50,7 @@
    :caption: Drivers
    :hidden:
 
+   drivers/d3d12
    drivers/freedreno
    drivers/llvmpipe
    drivers/openswr

docs/drivers/d3d12.rst

@@ -0,0 +1,49 @@
+D3D12
+=====
+
+Overview
+--------
+
+The D3D12 driver is a Gallium driver that emits API calls for Microsoft's
+:abbr:`D3D12 (Direct3D 12)` API instead of targeting a specific GPU
+architecture. This can be used to get full desktop OpenGL 3.3 support on
+devices that only support D3D12.
+
+Debugging
+---------
+
+There's a few tools that are useful for debugging D3D12, such as these
+environment variables:
+
+.. envvar:: D3D12_DEBUG <flags> ("")
+
+``verbose``
+   Enable verbose output to stdout
+``blit``
+   Trace blit and copy resource calls
+``experimental``
+   Enable experimental shader models feature
+``dxil``
+   Dump DXIL during program compile
+``disass``
+   Dump disassambly of created DXIL shader
+``res``
+   Debug resources
+``debuglayer``
+   Enable `debug layer`_
+``gpuvalidator``
+   Enable `GPU validator`_
+
+.. envvar:: DXIL_DEBUG <flags> ("")
+
+``verbose``
+   Enable verbose output to stdout
+``dump_blob``
+   Write shader blobs
+``trace``
+   Trace instruction conversion
+``dump_module``
+   dump module tree to stderr
+
+.. _debug layer: https://docs.microsoft.com/en-us/windows/win32/direct3d12/understanding-the-d3d12-debug-layer
+.. _GPU validator: https://docs.microsoft.com/en-us/windows/win32/direct3d12/using-d3d12-debug-layer-gpu-based-validation