Missing conventions

### Improvement proposal

The convention pages have the following TODOs

Code Conventions

* Javadoc
* APT - but as we are migrating, I think we can skip this
* Added by me: The import order only speaks of javax. I think this is useless nowadays with jakarta in place. My suggestion: Handle both, jakarta as well as javax, as any other 3rd party

Git:
* Gitingore

Locate the convention pages referenced in the issue and review their TODOs, especially the Code Conventions and Git sections. Check the existing guidance for Javadoc, APT, import ordering, and Git ignore rules. Done means the selected TODOs are addressed in the relevant convention pages, with the javax/jakarta recommendation resolved consistently.

Written by the indexing model from the issue text.