docs: link broker sites directly from the nav, add RabbitMQ

[powersemmi]

Description

The Brokers menu pointed each external broker (NATS, Redis) at an in-repo stub page whose
only content was a link to that broker's own documentation site, so reaching the docs took two
clicks. This makes the nav entries link straight to each broker's site and adds the RabbitMQ
broker (ruststream-lapin):

• Nav Brokers entries for NATS, Redis, and RabbitMQ are direct external links; the redundant
  brokers/nats.md and brokers/redis.md stub pages are removed.
• The brokers overview table gains a Docs column and a RabbitMQ row.
• The broker-switch comparison gains a RabbitMQ tab (LapinBroker::new(..)), one line from the
  others.

Memory stays an in-repo page (it is the in-core broker). Docs-only; the strict site build and the
fence lint pass.

Type of change

• Documentation (typos, code examples, or any documentation updates)

Checklist

• My code follows the project's style guidelines (just check passes: rustfmt, clippy, and
  cargo check with all features and with --no-default-features)
• I have performed a self-review of my own code
• I have made the necessary changes to the documentation (rustdoc and/or the docs site)
• My changes generate no new warnings (clippy runs with -D warnings)
• I have added tests that validate my fix or new feature
• New and existing tests pass locally (just test)