chore: revert "feat(gnoweb): add metadata to md content (#3797)"

[moul]

This reverts commit d1db75e (#3797).
See #3910.

Let's revert this now. Then we can try to improve SEO automatically by extracting the first H1 and the first paragraph, as this PR did manually. After that, we should take more time to consider aspects such as frontmatter and everything involved in allowing developers to specify "invisible" elements.

[Gno2D2]

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[alexiscolin]

@moul Should we keep these ones at least (default ones for source, actions, dir)? There is no UGC in those one (so no UGC risk)

[moul]

Sure, we can keep whatever you want. I tried to manage conflicts, but I'm not very familiar with this feature. I offer you two choices: continue working on my pull request to improve it, or we can merge this revert and you can focus on SEO optimizations later.