Add protocol media documentation guides

[samuel-williams-shopify]

Summary

• add a getting-started guide for media types, ranges, compatible objects, maps, and registry integration
• add a content-negotiation guide describing the protocol-http and application policy boundary
• generate Utopia Project agent context and README guide links

Verification

• COVERAGE=PartialSummary bundle exec bake test
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib
• bundle exec bake utopia:project:static output_path=/tmp/protocol-media-documentation force=true
• execute the documented protocol-http negotiation example