Document Spring Boot modules

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.

Start by reviewing the existing appendix, auto-configuration list, configuration properties list, and scattered auto-configuration customizer references. Map the Spring Boot 4.0 modules against those sources, then document each module's purpose, auto-configurations, properties, and customizers with clear parent references. Done means the module documentation is complete and reflects direct module usage.