Docs: Hint that `mdx.inline` is not transformed by Astro

[tordans]

This is a help out a bit with https://github.com/Thinkmill/keystatic/discussions/1318.

Ideally this section would go into more details how different frameworks (NextJS, Astro, …) treat the mdx.inline.

But this hint should still help out those that where looking for some tighter integration …

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Update: I see in https://github.com/Thinkmill/keystatic/pull/1260/files now that this change also need to happen on the markdoc page. – Let me know if this is fine and I can update the PR.

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