doc: Add documentation for runner.md under api

[gmuloc]

Description

Fixes #651

Reviewer questions

• Move scripts only in snippets?
  No - use examples
• Moving more stuff around?
  Not for now

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)