Versioned docs with mike #140

The docs site publishes one version, whatever is on dev. When 0.7 changes an API, the 0.6 documentation is gone, and anyone pinned to 0.6 has no accurate reference.

mike solves this. It publishes each version under its own path and adds a version dropdown to the header, and mkdocs-material supports it directly.

Deliberately deferred until the SDK settles. It restructures the Pages deploy in .github/workflows/docs.yml, and it only starts paying off once people are pinned to a released version. Filing it now so the reason it is not done is on the record rather than forgotten.

What it involves:

1. Add mike to docs-requirements.txt.
2. Add extra.version.provider: mike to mkdocs.yml.
3. Rework the deploy job to mike deploy against a gh-pages branch instead of the current artifact upload, with an alias so the bare URL lands on the latest release.
4. Decide what dev publishes as, and whether unreleased docs are visible at all.

Point 4 is the actual decision. The rest is mechanical.

Acceptance:

• Version dropdown appears and switches between at least two published versions.
• The bare docs URL resolves to the latest release.
• Releasing does not require manual steps beyond the existing tag flow.

Not a first issue.