doc(bpmn-model-api): add docs for bpmn model api

- how to create a new empty model instance
- how to navigate inside a model
- how to read/edit a model instance and attributes

related to #CAM-1703
fix(bpmn-model-api): use setDefinitions method in examples

related to #CAM-1703
doc(bpmn-fluent-api): add docs for fluent builder API
refactor(fluent-api): replace parallel method with new methods