fix(idr-api): replace mimeType enum with permissive pattern

The previous 15-item enum rejected the standard MIME types UNTP credentials use, including application/vc+ld+json (W3C VC 2.0) and application/linkset+json (RFC 9264 — already emitted by the resolver itself in its own Link header). Replaces the enum with a regex pattern that accepts any well-formed MIME type per RFC 6838, and updates the descriptions to call out the most common UNTP-relevant values. Regenerates the rendered idr-api.html docs to match.