docs: document a way to look at the logs

The logs are not easily accessible (see #779). You should provide some commands in the troubleshooting section to look at recent logs (since one does not always have a working log aggregation set up).

E.g.:
```
ls -rt /var/snap/charmed-postgresql/common/var/log/patroni | tail -100 | xargs -I{} cat /var/snap/charmed-postgresql/common/var/log/patroni/{}
```
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*Reported from: https://canonical-charmed-postgresql.readthedocs-hosted.com/14/reference/troubleshooting/cli-helpers/*