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

[via cfe-commits]

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+[clang-tidy/checks/**/*.rst]
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zeyi2 wrote:

It will work, I'll move it to that folder :)

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