Frees /products/ for the product page and makes the open-source archive
explicit, rather than leaving two sections that both look like "projects".
- Move the directory in both locales and update the 18 internal links
pointing into it.
- Rework helix for the GitHub layout: server/ and web/ are directories in
a single repository there, not the two separate repositories Gitea had.
The "fork both repositories" step and the section wording follow.
- Correct helix's stated Java version to 21, which is what the build
actually targets.