fix: terminology alignment and ADR 0007/0010 follow-ups

[liady]

Fixes #72.

Four small alignment items, none of them changing behavior on their own:

• The intro, the Catalog Entry terminology definition, and Design Goal 2 still described entries as identified by a "media type". ADR 0014 renamed the field to type precisely because it is an open type identifier, and the field definition already uses that framing, so the intro material now matches.
• Link relation discovery step 4 only accepted application/ai-catalog+json responses, while Location Independence makes serving that media type a SHOULD. A catalog served as application/json would pass the well-known flow but fail the link-relation flow. Step 4 now accepts generic JSON media types as long as the document has a valid specVersion.
• Added the "AI Card" terminology entry that ADR 0010 called for.
• ADR 0007 said the field is named url across all objects, but the spec uses uri where non-fetchable identifiers are allowed (attestation uri, governanceUri, registryUri, statementUri). Added an update note to the ADR recording that the split is deliberate, so implementers don't read it as drift.

[darrelmiller]

The value of type is still an IANA media type. We are sticking with this value domain because then we don't need to define our own syntax and range of valid values. There is precedence for using type as an indicator of media type in the IETF Web Linking specification https://datatracker.ietf.org/doc/html/rfc8288#section-3.4.1

[darrelmiller]

I'm not sure we really want to keep the terminology of AI Card. The repo was renamed and the org hasn't been changed primarily because we didn't want to break links. We have been using "catalog entry" as the primary term.