`ruff server`: Write a setup guide for Neovim (#10987)
## Summary
A setup guide has been written for NeoVim under a new
`crates/ruff_server/docs/setup` folder, where future setup guides will
also go. This setup guide was adapted from the [`ruff-lsp`
guide](https://github.com/astral-sh/ruff-lsp?tab=readme-ov-file#example-neovim).
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Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>
