[clang] [Doc][HLSL] Add documentation for root signature. (PR #83933)

[Xiang Li via cfe-commits]

python3kgae wrote:

> I think we need to break this PR up into different parts going to different places.
> 
> Documenting the RootSignature grammar and usage in the Clang docs makes sense.
> 
> I think documentation of the RootSignature binary format probably should live with the DirectX backend docs, since the code to generate the binary encoding should live in LLVM.
> 
> The last part of the document here about the implementation I think we should remove from this PR and bring that discussion over to https://github.com/microsoft/hlsl-specs. There are a lot of parts of the design that I'm not convinced are correct or lack appropriate detail for me to understand.
> 
> To @damyanp's point I think the discussion thread here has gotten long enough to be unwieldy. Rather than merging an incomplete document to Clang's docs, I think it would be better for us to iterate on the design outside the LLVM source tree and return to LLVM with a design we have a higher level of confidence in.
> 
> @python3kgae can you strip PR down to just the first parts that document Root Signatures and take the implementation discussion over to hlsl-specs?

Created https://github.com/microsoft/hlsl-specs/pull/193


https://github.com/llvm/llvm-project/pull/83933