feat(docs/downloads): card permalink hooks, date-sorted latest releases, content cleanup (#847)

- Add clickable permalink hook + selectable :target highlight to docs and
  download project cards; clicking copies the deep link, clicking the box
  selects it and updates the URL hash.
- Downloads 'Latest releases' now sorts projects by actual release date
  (parsing both 'Mon. DDth, YYYY' and ISO dates) and shows the 5 newest with
  dates; dropped the GPG/SHA-512/mirrors tag row.
- Horizon UI: drop the inaccurate 'Included in the main repo release' note
  (still on voting, not released).
- docs.yml: remove the SkyWalking Servers and Database descriptionItem notes;
  move SourceMarker to Ecosystem Retired Projects.