docs(v4): fill in missing variant field coverage

[ashleythedeveloper]

The v4.0.0 release-prep PR (#127) shipped without documenting three landings that are part of v4 but went unmentioned in the release notes and the docs site: the additive public and rel variant fields, and the MIME type loosening (the service now accepts any RFC 6838 well-formed media type, not the previous curated list). A canonical per-variant field reference was also missing entirely; the developer-guide described variants through a single registration example, leaving method, encryptionMethod, and registration-time accessRole undocumented since v2.0.0.

This PR fills those gaps without changing any code, version, or release tag. v4.0.0 already supports everything described here; the docs were lagging the binary.

What this PR changes

• documentation/docs/developer-guide/index.md: new Variant fields reference table covering every accepted field with type, required/optional, and contract notes. Includes the predecessor-version strip semantics on rel. MIME paragraph and prose intro updated for RFC 6838 acceptance and hreflang[]. Pointer to the Swagger UI as the authoritative DTO surface. Stale "itemDescription will be removed in v3.0" note corrected (the field remains accepted as an alias, confirmed by the existing e2e suite).
• documentation/docs/understanding-the-service/how-it-works.md: Language and MIME bullets in the variant-model section updated; new paragraph naming public, rel, accessRole, encryptionMethod, method and linking to the field reference.
• documentation/docs/migration-guides/v4.md: new ### Additive variant fields and MIME loosening subsection alongside the existing breaking-change subsections in "What changed".
• RELEASE_NOTES.md: peer subsection in v4.0.0's "What changed" block covering the same three additive landings.
• documentation/versioned_docs/version-4.0.0/...: mirror of every change above so the served 4.0.0 docs page matches the current docs. diff -rq documentation/docs/ documentation/versioned_docs/version-4.0.0/ is clean.

What this PR does NOT change

• version.json, app/package.json, documentation/package.json, documentation/versions.json, documentation/docusaurus.config.ts. v4.0.0 stays at v4.0.0.
• No app/ code. No new endpoints. No DTO changes.
• The :4.0.0 and :latest Docker tags are unchanged (they move on tag pushes, not master pushes).

Test plan

• yarn build in documentation/ (Docusaurus build clean, no broken links).
• diff -rq documentation/docs/ documentation/versioned_docs/version-4.0.0/ returns no differences.
• Field reference cross-checked against app/src/modules/link-registration/dto/link-registration.dto.ts and app/src/modules/link-registration/constants/untp-enums.ts (every DTO-accepted field is in the table; enum values for encryptionMethod and accessRole shorthand expansion are accurate).
• itemDescription alias claim verified against the existing e2e suite at app/test/link-registration/item-description-backwards-compat.e2e-spec.ts.