refactor(docs/bpmn20): add custom extensions doc structure
feat(docs/bpmn20): add camunda attributes documentation
feat(docs/bpmn20): wip camunda element documentation
feat(docs/bpmn20): add camunda attributes to task descriptions
feat(docs/bpmn20): add camunda attributes to event and subproess descriptions
fix multi instance loop characteristics
feat(bpmn20): add most camunda extension elements