[Docs][DirectX] Add relevant documentation of Root Signature (#149608)
This pr adds documentation of root signatures for reference of an LLVM
user.
The target audience is an LLVM user that wants to build a custom tool,
or front-end, that requires constructing a root signature part (RTS0)
for a `DXContainer`.
With that in mind, this pr adds documentation of the metadata
representation of root signatures implemented in LLVM and utilized in
Clang.