Add LSP documentation for Emacs

[sellout]

Overview

Documents how to configure Unison LSP on Emacs.

Interesting/controversial decisions

This only documents Eglot usage, not lsp-mode. Eglot is much simpler and is included with Emacs as of Emacs 29.

[sellout]

I don’t know if it shows up when other people view the PR, but my fork implicitly runs garnix CI, and it looks like some of the Nix packages aren’t currently building, so CI looks like it’s failing.

[aryairani]

@sellout No, it doesn't show garnix runs here.

Would you add yourself to CONTRIBUTORS.markdown too, acknowledging that your contributions are MIT-licensed. Sorry I forgot to mention earlier.