Restore the archive links the redesign dropped from Archived Releases

#431 added "- type: Archive" to releases.yml purely so the section got a nav
chip, and put the actual content in a hardcoded container-archived block in
downloads-block.html: a note that older releases are unmaintained but still
downloadable, plus links to the ASF archive and the incubator archive.

The redesign (#844) rewrote downloads-block.html and dropped that block, but
left the now-renamed "Archived Releases" entry in releases.yml behind. Since
that entry has never had a list, the chip and the <h2> have rendered above an
empty grid ever since.

Restores the text as the category description so it is data-driven rather than
hardcoded, and renders any category description under its heading. Every other
category has a null description, so nothing else changes.