docs: align the reference pages with what the framework does

[Lancetnik]

Split out of #3124, on top of the grammar pass (#3214, merged). Also carries the healthcheks.md → healthchecks.md rename with a redirect stub at the old URL (#3216, merged into this branch).

Every hunk here changes a statement, an identifier or an example, so this is the PR to read against the source. 62 files against main. By theme:

• Acknowledgement: Kafka/Confluent nack seeks the same consumer back instead of handing the message to another one; ack()/nack() are no-ops under ACK_FIRST; group_id is required for every other policy; SetupError fires when the subscriber is declared, not at startup; Redis streams with a consumer group are at-least-once; per-broker default policies; the shared no_ack include now describes AckPolicy.MANUAL.
• Message objects: which fields live on the wrapper and which on raw_message (Kafka, Confluent); decoded_body is gone and committed is an AckStatus (Redis).
• Examples that no longer run: faststream.rabbitmq → faststream.rabbit, @broker.handler → @broker.subscriber, Depends(cache=) → use_cache=, asyncapi_url → specification_url, HandlerNotFoundError → SubscriberNotFound, TestClient → the Test*Broker classes, the Kafka RPC how-to, Django asgi:application, datetime.UTC → timezone.utc, pel_testing.py include line ranges.
• Statements: Kafka publisher key is a per-publisher default that publish() overrides; batch publishing is not atomic; the ASGI CLI runs on uvicorn's supervisor, not gunicorn; NATS pull subscribers use an anyio task group; NATS scheduling argument names and the one-pending-schedule-per-subject rule; MQTT version defaults to 5.0; Prometheus broker label values; Redis RPC goes through request().
• Cleanup: the orphan dependencies/{class,global,sub,testing,yield}.md stubs, the pre-3.9 tab in the shared Annotated include, the >=0.4.0 install warning on the Confluent page.

Test plan

• just docs-check on this branch

The base branch was changed.