initial service examples

First YAML examples inspired by those recent conversations:

• https://github.com/valueflows/valueflows/pull/391#issuecomment-451949728
• https://github.com/valueflows/vf-apps-traversing-the-flows/issues/4#issue-395979472
• https://github.com/valueflows/valueflows/issues/290#issuecomment-447584697
• #386
• #392
• #300 (closed)