[3.x] Document using packaged Laminas checks

[kbond]

services:
    rabbitmq1:
        class: Laminas\Diagnostics\Check\RabbitMQ
        arguments: ...
        tags:
            - { name: 'liip_monitor.check', label: 'rabbitmq1' }
    rabbitmq2:
        class: Laminas\Diagnostics\Check\RabbitMQ
        arguments: ...
        tags:
            - { name: 'liip_monitor.check', label: 'rabbitmq2' }