[clang-tools-extra] [clang-tidy][NFC] Add `.editorconfig` for `.rst` files (PR #167269)

[Baranov Victor via cfe-commits]

vbvictor wrote:

> Closed due to the discussions in the [RFC](https://discourse.llvm.org/t/rfc-adding-editorconfig-for-clang-tidy-documentation/88849/4).
> 
> Thanks to everyone who participated in the PR review and the discussion.

We may add the config in the repo at the end, for now I just wanted to gather some feedback on the RFC from others.

https://github.com/llvm/llvm-project/pull/167269