Fix JavaDocs (and enable PlantUML)

[koppor]

First attempt to check for JavaDoc validity.

Adds PlantUML-taglet to enable PlantUML in JavaDoc. There is MermaidJS support for JavaDoc, thus I opted for PlantUML.

IntelliJ will eventualle get support for PlantUML as well: https://youtrack.jetbrains.com/issue/IDEA-315387 - please vote up

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

Not happy to havef syntax diagrams in javadoic

[koppor]

Wrong repository. Should have gone to @koppor

[koppor]

Follow-up: #10253

[koppor]

Discussion: Documentation should more go into the DevDocs. These could be linked from the JavaDoc.

We will include OpenFastTrace at #11710