doc: Add documentation for runner.md under api

[gmuloc]

Description

Fixes #651

Reviewer questions

• [ ] Move scripts only in snippets?
• [ ] Moving more stuff around?

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my code
• [ ] I have run pre-commit for code linting and typing (pre-commit run)
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes (tox -e testenv)