Discourage people from defaulting to VPN in the coordinator GUI

[AnotherKamila]

A VPN connection looks appealing to people because it pretends to be the easiest way, but in reality it's the most problematic. People in general are bad in assessing this kind of stuff, so we should direct them more and make it clear that this should only be a last resort.

It might also be a good idea to provide "VPN troubleshooting" documentation, so that users don't come directly to us with such problems.

[mkowalski]

Personally I believe the two points we have in https://docs.scionlab.org/content/faq/troubleshooting.html#vpn are clear enough to figure out what the problem is in case we have a user with VPN and SCION down. The problem is, so far I have not met a user who would go to this page before he calls us ...