docs: add versioned documentation

[geier]

Summary

• add VitePress documentation versioning with development and 1.9 snapshots
• add a path-preserving version switcher and centralized version metadata
• make /docs/ and /docs/latest/ redirect to the latest released docs without duplicating latest content
• generate version-prefixed sidebar links from one shared sidebar definition
• keep screenshots and shared docs assets in the root docs/_static/ folder instead of copying images into each version

Verification

• npm run build

Notes

• This PR currently uses vitepress-versioning-plugin@1.3.0 for versioned docs integration, but the plugin appears older / not very actively maintained and pulls in a nested vitepress@1.5.0 while this repo uses vitepress@1.6.4.
• The plugin-generated version switcher is disabled. The path-preserving version switcher, latest alias behavior, version metadata, and sidebar generation are owned by this repo.
• Because we already own most custom behavior, a follow-up may remove the plugin entirely and keep the same versioned folder structure manually if we want to reduce dependency risk.
• The release snapshot included here is temporary demo content to show the versioning feature. The actual release docs snapshot must be regenerated from the then-current development docs when the release is cut.
• bun.lock is untracked locally and was not included in this PR.