Document Spring Boot modules

[vpavic]

Following up on the modularization efforts in Spring Boot 4.0, I believe it would be important to provide adequate documentation about the newly introduced modules. For each module this should include:

• brief description that clarifies its purpose
• auto-configurations it provides (if any) coupled with:
  • configuration properties it provides
  • auto-configuration customizers it provides

Some of this information is already provided in the appendix, but in separate places - list of auto-configurations hints at what modules exist (but not all since some don't provide auto-configurations), while configuration properties list has grown quite large and would benefit from being documented in a way that reflects modularization. Various customizers are mentioned across the documentation, but not in a structured way that ensures all are listed with a clear reference to their parent auto-configuration.

My motivation here is that, with modularization in place, I'd like to move away from using starters in favor of using Spring Boot modules directly. In order to do so efficiently, documentation that provides information listed above is important, as based on that we can figure out which modules we need and what's the appropriate dependency configuration for each of them.

[wilkinsona]

Modularising the documentation, such that it's structured (partially at least), around the modules is something that we've discussed off and on over the past several months but we ran out of time in 4.0. In the meantime, there's some minimal information about the modules available in the migration guide.

Ideally, I'd like each module to contribute information to the docs and the documentation:spring-boot-docs project then becomes more of an aggregator and less of a module that knows about everything. It'll still have to know about everything to some extent to provide some ordering and structure but it could be significantly less than it is now.