Improve the documentation

[StroemPhi]

This PR further improves the current documentation to better explain the design principles and pattern of DCAT-AP+ as well as adding documentation on how to use DCAT-AP+ as basis for domain-specific DCAT-AP extensions.

[github-actions]

PR Preview Action v1.8.1
🚀 View preview at https://nfdi-de.github.io/dcat-ap-plus/pr-preview/pr-56/
Built to branch gh-pages at 2026-02-24 19:31 UTC. Preview will be ready when the GitHub Pages deployment is complete.

[StroemPhi]

@dalito, @HendrikBorgelt and @markdoerr could you please check the docs for errors? They might not be finished yet, but I think they are way better than before and thus a solid foundation for us to present DCAT-AP+ in other contexts in the future like the planned WG Onto presentation. Especially the updated versioning page needs your vetting @dalito.

Thank you

[HendrikBorgelt]

Documentation is looking good. I could not even find typos.

[dalito]

Very nice overhaul and good additions. I only have minor comments.

In live-preview of the documentation gnerated from this PR, the DCAT-AP+ logo is too small. It should at least double the size (or even more) so that the text can be easily read.

[StroemPhi]

Thank you @dalito will fix your suggestions later today.

[StroemPhi]

I could make the logo a bit bigger in docs\stylesheets\extra.css. If we want this better, we can tweak it there, causing the header to grow quite big though. Or we use another SVG. This one does not have a transparent background, but I doubt that it would loog good on this blue-greyish header we currently have.