manual: develop: mention link checker.

manual: update some URLs with redirects.

manual: intro: update icestorm link. The new link points to the same broken page, but at least now `pdm check` does not time out.

manual: factory→build

manual: contribute: fix syntax.

manual: intro: phrasing.

manual: index: add opengraph and twitter metadata. It is unclear whether services will follow the meta redirect on the landing /index.html page, but it seems the answer is "probably not". Be safe and add the metadata both on /index.html and /latest/index.html, for good measure.

CI: only clean gh-pages when deploying in the main repo.

docs/manual: new manual! Co-authored-by: Attie Grande Co-authored-by: Piotr Esden-Tempski Co-authored-by: Charlotte

meetings: add minutes from 2023-09-30