Add JDoc checks to PR pipeline

Checklist

• I am aware that this issue is being opened for the NewPipe Extractor, NOT the app, and my feature request will be dismissed otherwise.
• I made sure that there are no existing issues - open or closed - which I could contribute my information to.
• I have taken the time to fill in all the required details. I understand that the feature request will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.

Feature description

Check javadoc as part of the CI / PR pipeline to detect faulty docs before adding them to the dev branch.

Why do you want this feature?

Sometimes wrong JDoc is introduced in PRs. Format that is not accepted by the aggregatedJavadocs task lets the automated publishing of the docs fail which has been repeatedly happened after publishing a new version.

Additional information

An example PR fixing JDoc issues is #1356

Start by locating the CI/PR pipeline configuration and the aggregatedJavadocs task mentioned in the issue. Review PR #1356 for an example of the Javadoc problems this should catch. Done means pull requests check Javadoc before changes reach the dev branch and automated documentation publishing no longer fails because of faulty docs.