Add network connectivity checks to troubleshooting docs

[wojcik91]

Add a new page in troubleshooting section which would describe how to check network communication between various components:

• core -> proxy
• gateway -> core
• client -> gateway public IP
• connected client -> gateway VPN net IP
• connected client -> configured DNS server

[teon]

@kchudy this is part of the deployment we have talked about.

[kchudy]

Described in the docs https://docs.defguard.net/deployment-strategies/production-deployment-verification-guide