Align Accept-Language docs with strict negotiation and enforce the jsonapi_menu_items floor

Lauri Timmaneerequested to merge
issue/jsonapi_multilingual-3613673:3613673-readmecontributing-still-document into 1.0.x

README and CONTRIBUTING still described the pre- #3612442 implicit fallback ("never returns 404", "with fallback") and a stale uncacheable claim; both now document the strict ordered accept-list, the 406 with errors[0].meta.availableTranslations (live-verified), and a 406 curl example. The jsonapi_menu_items 1.2.8 floor moves from prose into composer require-dev plus a runtime hook_requirements warning, with the id-format dependency documented at the split site.

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

Closes #3613673

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