Document native retry mechanisms in message queues

Updates documentation across all relevant manual sections to explain the new
MessageQueue.nativeRetrial property and its impact on retry behavior:

- Add comprehensive "Native retry mechanisms" section to mq.md
- Update federation.md to note when retry policies are ignored
- Update send.md and inbox.md to explain native retry behavior
- Document which implementations currently support native retry
- Clarify benefits and usage of native retry mechanisms

The documentation now clearly explains when Fedify's retry logic is bypassed
in favor of backend-native retry mechanisms for better efficiency and
reliability.

Co-Authored-By: Claude <noreply@anthropic.com>