[clang-tools-extra] [clang-tidy][docs] Add documentation for Clang-Tidy Automation (PR #153166)

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

> You need to have `Sphinx` and build `docs-clang-tools-html` target in CMake (possibly using `ninja docs-clang-tools-html`). https://clang.llvm.org/extra/clang-tidy/Contributing.html#documenting-your-check

I believe you also need to enable these CMake flags:

```
-DLLVM_BUILD_DOCS=ON \
-DLLVM_ENABLE_SPHINX=ON \
```


https://github.com/llvm/llvm-project/pull/153166