Add troubleshooting guides

[Franky1]

Since the community forum is unfortunately full of posts about installation errors that occur again and again, it would be very helpful to have a kind of troubleshooting checklist to systematically check for all causes.

Instead of helping everyone in the forum with troubleshooting, you could instead refer to the Troubleshooting Checklist to identify the 90% most common problems.

This would also be a great help for those who are not very experienced in Linux administration and may not even know the necessary tools, e.g. netstat, ps, dig, ufw, tcpdump...

In this forum post there are a few suggestions for the checklist, which of course need to be extended:
https://community.jitsi.org/t/cannot-see-anyone-but-me-and-error-message-in-jvb-log-on-a-nated-vm-on-debian-10/59325/4

I think we would need one checklist for the "Easy Installation" and one for the Docker Installation.

[saghul]

Makes sense! Want to get started with a PR?