Major Reference Guide Restructuring/Refactoring [4.0]

[marleinevankampen]

Rewrite the chapters of the Axon Framework in the reference guide to reflect the logic in Axon 4.0.

The current version can be found on docs.axoniq.io. AxonIQ/reference-guide is the code base for the new reference guide.

As placing comments in the reference guide is not possible - hereby a list of ideas for changes

• Axon Framework Background -> Remove Terracotta / Gigaspaces reference
• Getting Started -> Is there a Gradle repository where we can link to?
• Configure Maven -> Mention the Spring Boot starter?
• Architecture Overview -> Is the SLF4J versioning still correct?
• Modules -> Is the setup of the modules still correct for 4.0?
• Modules -> axon-spring mention something about the starter here, or at least a reference to the Spring Boot starter example
• Commands -> Explain "wrapped using one of the CommandMessage implementations" a bit more.
• Event handling -> Rewrite to use the EventProcessingConfiguration instead of the EventHandlingConfiguration
• Saga -> Update with possibility to have multiple sagas in one event processor?
• Saga -> Quartz 2 still relevant?
• 1.1.3 Configuration API and [1.3.5] Spring Boot Auto Configuration](https://docs.axoniq.io/reference-guide/1.3-infrastructure-components/spring-boot-autoconfiguration) should link to each other or at least be in the same chapter.
• Event processing - Add information about replaying events