Add redirects for legacy ReadMe /reference/* API links

The old ReadMe.io API reference (developers.checklyhq.com/reference/<slug>)
404s for every deep link. Add 182 explicit redirects generated from the
frozen ReadMe-era slug map (lowercased operationId -> endpoint page), a
manual /reference/authentication entry, and a /reference/:slug* catch-all
to the API overview for slugs whose endpoints no longer exist.

mint broken-links never validates inbound-only redirect destinations, so a
new check (.github/scripts/check_redirect_destinations.py) runs in
static-docs-checks on any docs change and fails if a page rename/delete
dead-ends one of these redirects.

Note: developers.* traffic only reaches these redirects once the
docs.checklyhq.com Vercel shim forwards /reference/* paths (tracked
separately); direct www.checklyhq.com/docs/reference/* hits are fixed
immediately.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: sbezludny