docs: Add link back to Ruby LSP config documentation

[edwardloveall]

Small thing here, but there's no actual link back to the ruby-lsp docs, like there is for other languages or ruby LSPs (like solargraph). This should help users find the correct docs for configuring.

Release Notes:

• N/A

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[edwardloveall]

@cla-bot check

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[edwardloveall]

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[yeskunall]

OK, I see the motivation behind this: Shopify/ruby-lsp#3934 (comment).

[yeskunall]

@edwardloveall thanks for the sound change to our docs and congrats on your first PR contribution to Zed! Hopefully many more to come! 😁