Set up version-based documentation with mike

[Asgabani]

Fixes #122.

mkdocs.yml already declared extra.version.provider: mike (which enables the mkdocs-material version selector), but nothing actually deployed versioned docs: mike wasn't installed anywhere, and the release workflow ran a plain mkdocs gh-deploy --force, which overwrites the whole gh-pages branch with a single, unversioned build every release.

What this does:

• Adds mike to the docs dependencies.
• Replaces the gh-deploy step with mike deploy --push --update-aliases <major.minor> latest followed by mike set-default --push latest, deriving <major.minor> from the release tag (v0.1.2 → 0.1). One doc set per minor version, aliased to latest for the newest.
• Configures the git identity mike needs to commit to gh-pages, and explicitly fetches the gh-pages branch first (a checkout of the triggering ref doesn't include other branches — mike's own docs flag this as a common CI gotcha).

Also fixes a pre-existing, unrelated break in the same workflow: pip install .[docs] needs a [project.optional-dependencies] table, but docs was only defined under [dependency-groups] (PEP 735, likely for local uv use). That mismatch meant the deploy job's install step silently installed no docs dependencies — not even mkdocs itself. Added the matching [project.optional-dependencies] table so .[docs] actually resolves; left the [dependency-groups] entry as-is for any uv-based local dev flow.

Testing: validated pyproject.toml (TOML parse) and doc_deploy.yml (YAML parse + ruff clean on the repo). This workflow only runs on release: published, so it can't be exercised in CI on the PR itself — happy to walk through the exact mike commands with a maintainer if useful.