feat(docs-archive): add 1.6.0 and restore curated sidebars

[lakshay-nasa]

Adds 1.6.0 to the archive as the newest archived version (1.7.0 is current on docs.datahub.com).

Changes

• versioned_docs/version-1.6.0 from the release snapshot, sanitized; <Tab> → <Tabs> fixed in 3 files that otherwise fail the MDX compile
• Registered in versions.json / docusaurus.config.js, footer links updated
• Sidebar fix for 1.6.0 and 1.5.0 — both used the autogenerated fallback, which builds but renders a flat file listing instead of the nav the version shipped with. New scripts/convert-release-sidebar.js converts the release's sidebars.js snapshot properly and fails loudly if a doc id is missing. 1.3.0 and older were unaffected (native versioning wrote their sidebar JSON at the time).
• package-prebuilt.sh emits 308s from /docs/X.Y.Z/ to each version's landing page — version roots previously 404'd. 0.13.1 skipped, it has a real root page.
• docusaurus.single.config.js for single-version validation (~2 min vs ~27 min full build)

Notes

• 1.5.0's sidebar is a pre-existing bug fix, not scope creep — it's been wrong on the live site since it was archived.
• Already built and deployed: 14 versions, 6115 pages, all returning 200.