[llvm] [Doc][DirectX backend] Add documentation for root signature (PR #88795)

[Damyan Pepper via llvm-commits]

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@@ -0,0 +1,190 @@
+===================================
+Root Signature Serialization format
+===================================
+
+.. contents::
+   :local:
+
+Serialized format
+=================
+
+The root signature will be serialized into a binary format and saved in 'RTS0'
+part of DXContainer.
+The binary format is a sequence of bytes that can be used to create a root 
+signature object in the Direct3D 12 API. The binary format is defined by the
+`D3D12_ROOT_SIGNATURE_DESC (for rootsig_1_0)
+<https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ns-d3d12-d3d12_root_signature_desc>`_
+or `D3D12_ROOT_SIGNATURE_DESC1 (for rootsig_1_1)
+<https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ns-d3d12-d3d12_root_signature_desc1>`_
+structure in the Direct3D 12 API. (With the pointers translated to offsets.)
+
+It will be look like this:
----------------
damyanp wrote:

```suggestion
It will be like this:
```

https://github.com/llvm/llvm-project/pull/88795