Support Spring-Modulith-native extraction of Javadoc into Application Module Canvases

[odrotbohm]

Our documentation feature already supports including Javadoc extracted from the source code through the Spring Auto RESTDocs doclet. That, unfortunately, has been discontinued. Furthermore, it has always been a stretch having to set up a completely unrelated project to extract the metadata. It also suffered from not properly supporting overloaded methods.

To improve the situation, we should …

• … create a Spring Modulith-native way of extracting Javadoc from the code base. Spring Boot uses an APT to extract Javadoc from configuration property fields. We should be able to do the same, but rather inspect all types and methods. The code creating the Application Module Canvas should be reviewed to make more use of that metadata.
• … fold in the metadata-loading parts of Spring Auto RESTDocs to avoid the additional test scope dependency. As the project is not maintained anymore, it's safe to assume no further adjustments to their code base. We should officially deprecate the support in the form of a warning log if a Spring Auto RESTDocs configuration file is found.