feat(docs/bpmn20): add camunda custom extensions

* create new section for custom extensions
* add subsection for camunda extension elements
* add subsection for camunda extension attributes
* add camunda extension subsection to every BPMN element with extensions

related to #CAM-1854
chore(reset): add docs about HistoricTaskQuery

related to #CAM-1875
docs(cockpit/plugin): add instructions for client side plugin exclusion
fix(bpmn20): fix typo