feat: add multi-version docs support

[schacherle]

Changes

Enables Docusaurus's native versioned docs so users can view documentation for a specific released version of Shipwright Build, instead of only ever seeing the latest unreleased state.

• Cuts the first versioned snapshot at 0.20, matching the current latest Build release line (v0.20.x).
• Uses Docusaurus's default versioning behavior: the unprefixed /docs/ path serves the latest released version (0.20), and the unversioned docs/ folder (tracking main, i.e. unreleased work) serves at /docs/next/. This matches the convention used by Jest, React Native, and Docusaurus's own docs site.
• Adds a version dropdown to the navbar (right-aligned, before the GitHub link — same placement as docusaurus.io's own site).
• Documents the versioning workflow in the README for future version cuts.

Note on sequencing vs. SHIP-0041

I'm aware of the (still provisional, unimplemented) SHIP-0041 docs restructure proposal. Cutting a version now means this first 0.20 snapshot stays in the current build/getting-started/ref layout even if/when that restructure lands later — subsequent versions cut after a restructure would just pick up the new layout. This is the same tradeoff every project with versioned docs eventually hits (older versions naturally look structurally different from current docs), so I don't think it should block this from landing. Happy to hold off if maintainers feel otherwise.

Related Issue

Fixes #136

Type of PR

/kind feature

Submitter Checklist

• Includes tests if functionality changed/was added
• Includes docs if changes are user-facing
• Kind label has been set
• Release notes block has been filled in, or marked NONE

Release Notes

Documentation is now versioned. The `/docs/` path serves the latest released version, and in-progress/unreleased docs are available at `/docs/next/`.

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[adambkaplan]

/lgtm

I love it!

Regarding sequencing - since we just announced v0.20, I'd rather add multi-version docs support now. This has a substantial benefit since adopters and users will see the "released" docs by default, giving us flexibility to modify structure for the "next" release.

[adambkaplan]

This update also provides a nice warning if a user sees the "next" docs currently under active development:

[IrvingMg]

@shipwright-io/approvers could someone please take a look?