S23.07: Landing page and version switcher

[DavidCozens]

Parent epic: #246

Goal

Give users a way to discover and switch between published versions of
the docs.

Tasks

• Add a small landing page at the root of gh-pages that:
  • Lists all available versions.
  • Links to each.
  • Highlights /latest/.
  • Makes clear which entry is main (development, may change) vs
    tagged stable releases.
• Configure Sphinx's version-aware navigation so that within any
  given version, the user can see they're on v1.0.0 (or main)
  and switch to other versions.
• Mechanism: a generated versions.json at the root of gh-pages,
  consumed by a small JS snippet in the theme footer (or theme-native
  version selector if furo / sphinx-book-theme provides one
  natively). Keep it simple — no need for anything sophisticated.
• The landing page and versions.json must be regenerated on each
  release (S23.05) and on each push to main (S23.04) so they stay in
  sync with what's actually published.
• Update DEVLOG.md.

Done when

• Visiting the Pages root shows a landing page listing all versions.
• /latest/ is visually highlighted and labelled as such.
• main is visibly distinguished from tagged versions.
• From any version, users can switch to any other version via the
  version selector.
• versions.json is regenerated by both the S23.04 and S23.05
  workflows.