feat(api-docs): version API reference

[imdhemy]

Summary

• introduce an explicit versioned-pages mechanism for auxiliary versioned routes
• move the API reference to canonical /docs//api-reference routes and keep /api-docs as a compatibility redirect
• version OpenAPI sources and generated outputs per docs release while moving OpenAPI build scripts under /openapi

Behavior Change

• the homepage no longer shows a global API Reference link
• versioned docs pages now expose an API Reference link in the left navbar that stays within the active docs version
• each published docs version resolves to its own OpenAPI source file and generated API reference output

Review Focus

• verify the separation between docs registry concerns and the new opt-in versioned-pages mechanism
• verify the API navbar visibility and routing behavior on homepage, docs pages, and versioned API pages
• verify the OpenAPI generation contract now uses openapi/.yaml and static/openapi// consistently

Notes

• validation completed with npm test, npm run build:api, npm run lint, and npm run build
• openapitools.json was removed because the versioned build path no longer relies on that config file
• Closes feat(api-docs): align API reference with docs versions #28