Documentation: finish API documentation migration and install SpringDoc

Description

The current API docs, still named "proposal", really need to be replaced. Many controller actions already have javadoc, but not all, and we need a tool to present this documentation in a nice, REST-y way.

Priority

Medium -- The current docs are still workable, as they are accurate and are presented in an acceptable way.

Definition of done

• - All controller actions are properly documented, complete with input constraints, output specification, error cases and examples
  • - The no-javadoc Checkstyle suppression has been removed
• - A SpringDoc installation is serving the documentation from the core
• - The old docs have been removed