* Add spec generation to offload-tblgen tool
* This patch adds generation of Sphinx compatible reStructuedText
utilizing the C domain to document the Offload API directly from the
spec definition `.td` files.
* Add Sphinx HTML documentation target
* Introduces the `docs-offload-html` target when CMake is configured
with `LLVM_ENABLE_SPHINX=ON` and `SPHINX_OUTPUT_HTML=ON`. Utilized
`offload-tblgen -gen-spen` to generate Offload API specification docs.
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