merge more "onionprobe alerting rules from upstream"

The following discussion from !66 (merged) should be addressed:

• @rhatto's comment says:

Thanks!

With this feedback in mind, I have improved Onionprobe alerts and tests as part of tpo/onion-services/onionprobe#102 (closed). Both rules and tests should now be more self-explanatory.

Also implemented:

• A new alert for tracking issues with Onionprobe itself.
• A troubleshooting page in the docs.

So now you could consider adding the other tests that are relevant to your use case.

let's see what we can reuse from there, now that upstream alerts and tests are clearer and available!