[llvm] [DirectX][DXIL] Design document for TableGen Spec of DXIL Operations (PR #85170)

[Jay Foad via llvm-commits]

jayfoad wrote:

@bharadwajy this seems to break the docs build with:
```
[1/2] Generating man Sphinx documentation for llvm into "/home/runner/work/llvm-project/llvm-project/llvm-build/docs/man"
FAILED: docs/CMakeFiles/docs-llvm-man /home/runner/work/llvm-project/llvm-project/llvm-build/docs/CMakeFiles/docs-llvm-man 
cd /home/runner/work/llvm-project/llvm-project/llvm-build/docs && /usr/local/bin/cmake -E env /opt/hostedtoolcache/Python/3.11.9/x64/bin/sphinx-build -b man -d /home/runner/work/llvm-project/llvm-project/llvm-build/docs/_doctrees-llvm-man -q -t builder-man -D version=19 -D release=19.0.0git -tPreRelease -W /home/runner/work/llvm-project/llvm-project/llvm/docs /home/runner/work/llvm-project/llvm-project/llvm-build/docs/man

Warning, treated as error:
/home/runner/work/llvm-project/llvm-project/llvm/docs/DirectX/DXILOpTableGenDesign.rst:document isn't included in any toctree
```
See for example: https://github.com/llvm/llvm-project/actions/runs/9306373243/job/25615439087?pr=93801

https://github.com/llvm/llvm-project/pull/85170