Re-order and condense extension metadata docs

[holly-cummins]

The extension-metadata guide is a bit of a franken-guide, with part of it intended for tooling authors, and part of it intended for extension authors. Normally, this is ok, because the most important bit of the guide for authors was at the top. That changed recently, so this PR:

• Moved the section about the file users need to edit to first in the document, so it's easy to find
• Tried to clarify and condense the discussion of file types at the beginning
• Removed the generated yaml example, which is no longer preferred, and moved its annotations to the generated json example

Deep link to preview: https://quarkus-pr-main-56490-preview.surge.sh/version/main/guides/extension-metadata/

[github-actions]

🎊 PR Preview 67076a3 has been successfully built and deployed to https://quarkus-pr-main-56490-preview.surge.sh/version/main/guides/

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[geoand]

Ah, this now needs a rebase onto main

[aloubyansky]

I'm sure it was me who messed it up for you. Good luck reviewing my cpe-artifacts compression paragraph.

[holly-cummins]

I think it was #55762, so not too bad to merge. A lucky escape. :)

[quarkus-bot]

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It should be safe to merge provided you have a look at the other checks in the summary.