[clang-tools-extra] [clang-tidy][NFC] add '.clang-tidy' config for clang-tidy project (PR #147793)

[Baranov Victor via cfe-commits]

vbvictor wrote:

> One last thing I'm missing: can you update the clang-tidy contributors guide, to explain that we expect people to run clang-tidy as well (and what command to run)? Since it's not currently enforced in CI.

I will add it in a separate PR. Don't feel right to make kind-of unrelated changes after most of the people gave their consent.

https://github.com/llvm/llvm-project/pull/147793