docs(messaging): connecting to an external ActiveMQ broker

[iliyan-velichkov]

Documents the messaging broker configuration added in
eclipse-dirigible/dirigible#6870:

DIRIGIBLE_MESSAGING_BROKER_URL=tcp://activemq:61616
DIRIGIBLE_MESSAGING_BROKER_USERNAME=...
DIRIGIBLE_MESSAGING_BROKER_PASSWORD=...

Unset keeps today's behaviour - the broker embedded in the server process.

Why more than one page changed

The env-var reference carried no messaging section at all (DIRIGIBLE_MESSAGING_USE_DEFAULT_DATABASE
existed only in the archived help_old tree), and several pages asserted the broker is always
embedded. The producer SDK page went as far as promising "no broker URL, no client configuration",
which is now wrong.

New content

• help/reference/environment-variables - a Messaging section (between Flowable and CMS/S3), all
  four variables with defaults, plus the two operational consequences.
• help/setup/environment-variables - the install-oriented subset of the same.
• help/develop/message-listeners - a Broker - embedded or external section: the configuration,
  ssl:// and failover:(...) URLs, a tip on why a shared broker is what makes a global:
  destination a contract between deployments, and what differs in external mode.
• help/ide/perspectives/messaging - an Embedded broker only section. The perspective reads the
  in-process BrokerService (JMX is disabled), so against an external broker its endpoints are not
  registered and answer 404.

Corrections to existing statements

sdk/messaging/producer, sdk/messaging/index, help/artefacts/process/listener,
help/ide/views/listeners, help/reference/glossary - each said or implied the broker is always
embedded. Reworded to "the platform's / the runtime's broker" with a link to the new section. No
change of meaning for a default deployment.

help_old/ was deliberately left alone (archived, out of nav).

Verification

• npm run docs:build completes clean.
• The three new anchors resolve in the built HTML (#broker-embedded-or-external, #messaging,
  #embedded-broker-only), so the cross-links between the pages are live.
• Style rules followed: VitePress ::: containers, no em-dashes or en-dashes, absolute cross-links,
  frontmatter intact. No config.mts change needed - every page edited is already in the nav.

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