Add versioned DocC publishing

[Kyle-Ye]

Summary

• add a .vdc.json configuration that publishes the development branch and the latest two release series
• upgrade the existing GitHub Pages workflow from a single DocC archive to a cached VersionedDocC build
• add version selection, version-correct GitHub source links, and adjacent-version public API changes
• update the README to use the stable versioned GitHub Pages URL

The release list is selected dynamically with releasePolicy.latest: 2; it currently resolves to main, 3.0.0, and 2.7.0 without hard-coding release tags in the configuration.

Why

The existing Pages workflow only publishes the current main documentation. VersionedDocC keeps immutable release documentation available under stable URLs and generates API comparisons from the releases' public symbol graphs.

Preview

• Versioned documentation
• API changes
• Reference fork

Validation

• built all three documentation versions locally with VersionedDocC 0.0.13
• verified the version selector, API comparison selector, and GitHub source link in a local browser preview
• verified 2.7.0 → 3.0.0 produces 78 added, 17 modified, and 37 removed public API entries
• deployed the fork through the updated workflow: https://github.com/DocCLab/MarkdownView/actions/runs/31041404655
• confirmed the fork's root, Changes page, and all three documentation versions return HTTP 200

[LiYanan2004]

@Kyle-Ye Thanks!