Delete the obsolete user guide for slang-gfx

[jkwak-work]

When I asked about slang-gfx, Cursor found information from an obsolete document that is no longer valid.

docs\gfx-user-guide\index.md

The use of slang-gfx is no longer officially supported in the favor of slang-rhi.
We should delete the directory with all files inside of it.

[aidanfnv]

I noticed that the README contains a reference to those docs:

These examples use a graphics layer that we include with Slang called "GFX" which is an abstraction library of various graphics APIs (D3D11, D2D12, OpenGL, Vulkan, CUDA, and the CPU) to support cross-platform applications using GPU graphics and compute capabilities. 
If you'd like to learn more about GFX, see the [GFX User Guide](https://shader-slang.com/slang/gfx-user-guide/index.html).

Is that no longer true? Should it be replaced with a reference to slang-rhi, or should just the reference the gfx-user-guide be removed?