[DOC] Replace 'Diagram usability' Github label/Milestones by several Github labels/Milestones

[csouchet]

To be broken down in terms of features:

• labels
  • [x] BPMN diagram navigation (defined by #1260)
  • [x] BPMN diagram overlays
  • [x] BPMN diagram styling
  • we have decided to keep the BPMN diagram usability label for issue/PR that don't match the new and more precise labels
• milestones
  • [x] add new Overlays first improvements milestone and move remaining overlays issue there
  • [x] for now, we keep the general milestones about BPMN diagram usability as we don't want to introduce or spend more time on milestones reorganization

⚠️ Modify the release drafter (github actions): done in https://github.com/process-analytics/bpmn-visualization-js/pull/1311

[tbouffard]

All tasks completed, labels updated.