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

[via cfe-commits]

EugeneZelenko wrote:

> > But what about C/C++/Python/CMake files?
> 
> For C/C++ we have `clang-format`. For CMake we have strategy "just make look like everywhere else" so I don't see how we can use different style of Cmake files in `clang-tidy`

`insert_final_newline` seems reasonable setting for all kinds of files - it's often missing in C/C++ files. `max_line_length` (if shown in editor as vertical line, like Visual Studio Code does) is also helpful.

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