Add markdown-ts-mode to the Emacs markdown configuration documentation

As of Emacs 31, `markdown-ts-mode` is now a core feature and the documentation points only to the older, non-tree-sitter `markdown-mode`. For people who use later Emacs to enjoy Harper without fuss in Markdown buffers, it would be helpful to include the new mode name. Perhaps someone can update https://github.com/Automattic/harper/blob/master/packages/web/src/routes/docs/integrations/emacs/%2Bpage.md to include that. Thank you.

Open packages/web/src/routes/docs/integrations/emacs/+page.md and read the existing Markdown integration guidance. Update the documentation to include markdown-ts-mode alongside the older markdown-mode reference. Done means users of Emacs 31 can find the core mode name when configuring Harper for Markdown buffers.

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