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
astral-sh/ruff-lsp#19 (comment))


## Test Plan

I've use this setting myself. And I will continue maintain this part as
long as I use it.

---------

Co-authored-by: Dhruv Manilawala <[email protected]>

docs/editors/setup.md

@@ -333,6 +333,17 @@ IntelliJ Marketplace (maintained by [@koxudaxi](https://github.com/koxudaxi)).
 
 ## Emacs
 
+Ruff can be utilized as a language server via [`Eglot`](https://github.com/joaotavora/eglot), which is in Emacs's core.
+To enable Ruff with automatic formatting on save, use the following configuration:
+
+```elisp
+(add-hook 'python-mode-hook 'eglot-ensure)
+(with-eval-after-load 'eglot
+  (add-to-list 'eglot-server-programs
+               '(python-mode . ("ruff" "server")))
+  (add-hook 'after-save-hook 'eglot-format))
+```
+
 Ruff is available as [`flymake-ruff`](https://melpa.org/#/flymake-ruff) on MELPA:
 
 ```elisp