display rule graph instead of DAG

- add script `tests/test_rule_graph/test.sh` to generate a rule graph in `images/rule_graph.svg`
- display rule graph created in `README.md` instead of specific workflow DAG
- add test script to GitLab CI config
- renamed test to create workflow DAG from `test_create_dag_chart` to `test_create_dag_image` (also output file is renamed from `images/workflow_dag.svg` to `images/dag_test_workflow.svg`