Document editors that support MDX language server

[remcohaszing]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
• If applicable, I’ve added docs and tests

Description of changes

Both lsp-mode and nvim-lspconfig support the MDX language server.

Refs #474
Refs https://www.reddit.com/r/neovim/comments/1ewwtok/has_anyone_figured_out_intellisense_in_mdx_files/

[changeset-bot]

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[github-actions]

Hi! This is accepted and can go somewhere!

Team: please review this PR and use the area/* (to describe the scope of the change), platform/* (if this is related to a specific one), and semver/* and type/* labels to annotate this.

[wooorm]

Did you test both? Seems good to me but I have no experience with em