feat: implement autodoc plugin

[paullatzelsperger]

What this PR changes/adds

This PR implements a Gradle plugin that does two things:

• adds an annotationProcessor dependency on the autodoc-processor module to the Gradle project. Whenever the compileJava task is executed, the manifest gets generated.

• registers a task named autodoc, that does nothing except express a dependency onto the compileJava task. That way, the manifest can get regenerated by calling ./gradlew autodoc

Why it does that

to have the autodoc plugin readily available as build script dependency

Further notes

• The Maven coordinates of the processor module is currently hard-coded. At least the version should be resolved dynamically.
• the compiler args are resolved automatically. Especially the output location is not yet configurable. That should be fixed.
• More tasks (such as printModuleConfig) can be added later

Note to reviewers: lets use this draft as basis for an early discussion. Polishing (where needed) comes later :)

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• assigned appropriate label? (exclude from changelog with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)