docs: align public guidance with v2 release candidate

[seanbrar]

Summary

Aligns public documentation with the shipped v2 release candidate: main now installs the prerelease explicitly, links to the versioned next docs instead of stable v1 docs, and describes the v2 API as shipped in RC rather than merely planned. It also fixes docs workflow behavior so concurrent PR previews do not cancel each other and prerelease tags retain their full version without taking the latest alias.

Related issue

None

Test plan

• just check — Ruff, rumdl, mypy, and 450 non-API tests passed.
• uv run mkdocs build --strict --clean — strict docs build passed.
• actionlint .github/workflows/docs.yml — passed.
• Exercised the tag-version shell logic for v2.0.0-rc.1, v2.0.0, and v1.8.1; results were respectively 2.0.0-rc.1 with no aliases, 2.0 with latest, and 1.8 with v1.
• Verified git diff --check passes.

Notes

The GitHub github-pages environment previously allowed only main, which rejected the v2.0.0-rc.1 tag deployment and would also reject v1. Its deployment policies have been updated separately to allow only main, branch v1, and tags matching v*.

The failed RC deployment was protective in hindsight: the old workflow would have shortened v2.0.0-rc.1 to 2.0.0-rc and promoted it to latest. This change prevents prerelease tags from changing the stable docs default.

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• PR title follows conventional commits
• just check passes
• Tests cover the meaningful cases, not just the happy path
• Docs updated (if this changes public API or user-facing behavior)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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