Add Eglot setup guide for Emacs editor (#12426)
## Summary
The purpose of this change is to explain how to use ruff as a language
server in Eglot with automatic formatting because I've struggle to use
it with Eglot. I've search it online and found that there are some
people also struggle too. (See [this reddit
post](https://www.reddit.com/r/emacs/comments/118mo6w/eglot_automatic_formatting/)
and
https://github.com/astral-sh/ruff-lsp/issues/19#issuecomment-1435138828)
## Test Plan
I've use this setting myself. And I will continue maintain this part as
long as I use it.
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Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>
