Reorder docs (cluster, persistent entities, broker,...)

[ignasi35]

Instead of:

Writing persistent and clustered services

    Persistent Entity
    Cassandra Persistent Entities
    Relational Database Persistent Entities
    Persistent Read-Side
    Cassandra Read-Side Support
    Relational Database Read-Side Support
    JDBC Read-Side Support
    JPA Read-Side Support
    Publish-Subscribe
    Serialization
    Cluster

Decouple services with a message broker

    Message Broker Support
    Message Broker API
    Lagom Kafka Client
    Message Broker Testing

Mode to something like:

Clustered services 

    Cluster (includes a small intro page about clusters in Lagom and presents bootstrapping, management, projections, clustered services, etc... cros-linking to forward sections)
    Cluster Setup (this page would contain the current content of https://www.lagomframework.com/documentation/1.5.x/java/Cluster.html and is all technical details about formation, discovery, etc...) 
    Serialization
    Projections (new page introduces ReadSide Processors and TopicProducers) 
    Publish-Subscribe

Writing persistent and clustered services

    Persistent Entity (add a x-ref to serialization and cluster setup)
    Cassandra Persistent Entities
    Relational Database Persistent Entities
    Persistent Read-Side (add a x-ref to projections, serialization and cluster setup)
    Cassandra Read-Side Support
    Relational Database Read-Side Support
    JDBC Read-Side Support
    JPA Read-Side Support

Decouple services with a message broker

    Message Broker Support 
    Message Broker API - Producer (add a x-ref to projections, serialization and cluster setup)
    Message Broker API - Subscriber 
    Lagom Kafka Client
    Message Broker Testing

Note that Message Broker API is already quite big so I'd split it in two parts (production and subscription)

[ignasi35]

related to #2049