Add docs around recovering from a failed firewall.

[marrobi]

If the TRE firewall has to be deleted, or gets deleted by terraform but not recreated, then it can be hard to recover as the resource processor has no internet access.

To resolve the route table, rt-<tre_id> for the ResourceProcessorSubnet needs removing temporarily to allow internet access, then the firewall shared service can be updated/upgraded so that the firewall is recreated. Then the route table needs putting back on the ResourceProcessorSubnet.