miscellaneous doc enhancements

[eroell]

PR Checklist

• [x] This comment contains a description of changes (with reason)
• [x] Referenced issue is linked fixes #811
• [ ] If you've fixed a bug or added code that should be tested, add tests!
• [x] Documentation in docs is updated

Description of changes Some doc enhancements, see linked issue

Technical details

Additional context

[Zethson]

Not the sexiest of all icons ^_^ Maybe something like https://thumbs.dreamstime.com/b/therapy-journey-rgb-color-icon-taking-medications-treatment-private-psychotherapy-practice-health-problem-remediation-204816600.jpg? I did not check copyright

[Zethson]

We could rename usage.md to API.md