docs: add Integrations catalog reference page

[strawgate]

Adds a Reference → Advanced → Integrations page documenting the integrations catalog — one-click dashboards + alerts for Redis, PostgreSQL, MySQL, MongoDB, Elasticsearch, Memcached, RabbitMQ, Kafka, NGINX, and Apache.

Covers:

• What an integration delivers — a standard dashboard, live active alerts, the OTel Collector setup config, and on-demand detection.
• How to install — Detect and Install (whole catalog) or Install content (one pack); attaching a notification channel.
• The catalog — Available / Detected / Installed status, filters, search.
• Access from the MCP server (integration_list / integration_install).
• The available integrations.

Pairs with the platform PR that adds the feature (pydantic/platform#24849), which is gated behind the integrations_catalog flag — hold merge until the feature is enabled for users.

Catalog hero screenshot captured from a local stack; happy to re-shoot at a specific viewport if preferred.

[cubic-dev-ai]

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_{Re-trigger cubic}