document abra commands for managing secrets and for checking logs

[dwaxweiler]

What does this PR do?

I have added some abra commands for managing secrets and for checking logs, which I learned when deploying an own Karrot instance. Others might find them useful too.

Checklist

• [ ] added a test, or explain why one is not needed/possible...
• [X] no unrelated changes
• [x] asked someone for a code review
• [ ] joined #karrot:matrix.org
• [ ] added an entry to CHANGELOG.md (description, pull request link, username(s))
• [ ] tried out on a mobile device (does everything work as expected on a smaller screen?)

[dwaxweiler]

@nicksellen, could you review this please?