feat: directly serve library document markdown

[SeanCassiere]

Redo of #396 with the new changes available using the features in the TanStack Start Alpha channel.

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Updated description from #396.

This is a POC PR that lets the user to access the markdown documentation pages by adding .md to the end of the path.

• /form/latest/docs/framework/react/guides/basic-concepts.md serves the markdown version of the documentation page.

Why is it useful?

This feature is useful for using the documentation with LLMs, and was inspired by vite:

For every page you can get the original markdown by replacing the .html extension with .md - e.g.:

https://vite.dev/guide/rolldown
(Posted by Evan You)

This feature would let the user to pass the documentation pages as context to the LLM-s they use for specific versions of tanstack libs, which would make sure that they always get the right information, even if there were breaking changes between versions.

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