Antora docs site has no version selector — pin per release instead of a single unversioned component #317

Summary

`docs/antora.yml` declares `version: ~` — Antora's syntax for an explicitly
unversioned component. The published site (this repo's `docs.yml` deploys on every
push to `develop`, so it's at least always current — unlike the engine repo, see
SKaiNET-developers/SKaiNET#975) only ever shows the latest `develop` snapshot, with no
way to view the docs as they were for an older release (e.g. what the BOM version pins
looked like at 0.39.1 vs current). Companion issue:
SKaiNET-developers/SKaiNET#976 (engine repo, same gap, same Antora setup/Docker image).

Proposed direction

1. Set `version:`/`display_version:` in `docs/antora.yml` per release instead of `~`
   — this repo already has a `release: prepare X.Y.Z` commit convention (see e.g. the
   0.40.1/0.40.2 release-prep commits), a natural place to bump it alongside
   `VERSION_NAME` in `gradle.properties`.
2. `antora-playbook.yml`'s `content.sources` adds historical tag refs alongside
   `branches: develop`, so each tagged version gets its own page tree and the site
   gains a version dropdown.
3. Decide retention policy (all tags vs. last N minor versions).

See SKaiNET-developers/SKaiNET#976 for the fuller sketch — same shape applies here.