[clang-tools-extra] Add clang-tidy external examples (PR #106675)

[Congcong Cai via cfe-commits]

HerrCai0907 wrote:

You can add `toctree` in `clang-tools-extra/docs/clang-tidy/index.rst`
example commit: https://reviews.llvm.org/D54945

https://github.com/llvm/llvm-project/pull/106675