DO NOT MERGE: Add reference doc for crJSON

[crandmck]

HOW THIS WILL BE DOCUMENTED IS UNDER DISCUSSION. DO NOT MERGE.

Add CrJSON schema reference documentation

Adds a reference doc for the CrJSON schema (crJSON-schema.json) using the existing SchemaReference component, following the same pattern as the Builder/Reader/ManifestDefinition/Settings references.

Changes:

• Fix SchemaReference.js to correctly resolve $refs using the older definitions keyword (#/definitions/...), not just #/$defs/... Needed because CrJSON's schema predates the schemars-generated schemas and wasn't written with $defs.
• Add docs/manifest/json-ref/crjson-ref.mdx and wire it into the sidebar and SDK object reference index.
• Add crJSON-schema.json to static/schemas/ and register it in remote-docs.json so it's fetched automatically from contentauth/c2pa-rs@main on every install/build (same mechanism already used for sb-alg-list.json), keeping it in sync without a dedicated GitHub Actions workflow.

Note: Unlike the schemars-derived schemas (synced only on tagged c2pa-rs releases via update-schemas.yml), this schema tracks c2pa-rs@main continuously since it's a hand-authored fixture with no release binding.

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[lrosenthol]

You CANNOT copy the crJSON schema from the C2PA site. Please reference it, not copy it!

[crandmck]

You CANNOT copy the crJSON schema from the C2PA site. Please reference it, not copy it!

As discussed separately, this NOT a copy from the C2PA site, but is an ephemeral copy from https://github.com/contentauth/c2pa-rs/blob/main/sdk/tests/fixtures/schemas/crJSON-schema.json updated via workflow.

It sounds like this file should not exist, but then we need a canonical version that the SDK uses/supports, since that might not always be the same as what's in the C2PA spec.