docs: expand thin provider READMEs and complete Related cross-links

[tgiachi]

Summary

Phase 3 (final) of the docs improvement plan - content depth.

• 6 provider READMEs expanded to the Crypto/Vfs quality bar (82-127 lines each): Messaging, Caching, Messaging.RabbitMq, Persistence.MessagePack, Templates, Workers. Every technical claim verified against the source (RabbitMq quorum queues/broker-side DLQ/recovery, MessagePack contractless resolver + Keep-based serializer override, Workers YAML config with real defaults incl. the machine-name WorkerId fallback, the 4 dotnet-new templates with their real options). All bootstrap snippets use the 0.15 explicit pattern.
• Related sections completed: added to the 5 READMEs missing them (Tui, Aws.Abstractions, Persistence.Abstractions, Telemetry.OpenTelemetry, Telemetry.Abstractions) plus MessagePack; every package README now cross-links its article/tutorial/siblings with absolute site URLs. All 26 link targets verified to exist.

READMEs render on GitHub, NuGet and the docs site (via include) - no relative repo links, no mermaid, no em dashes.

Test Plan

• docfx build 0 warnings; all Related link targets exist
• Reviewer spot-checked the riskiest claims against source (8/8 PASS); one snippet using fixed before merge
• Text-only change - no code touched