Air gapped documentation do not handle the case where we have no default route

Bug Report

Description

In some Air-gapped environments, we may not have any default routes configured.

But by default, Talos checks that there is one for the connectivity check (it does not prevent installing/running the cluster but the node will never become ready and keep the connectivity check as failed)

Workaround

As a workaround (that was a bit tricky to find, no documentation about it I had to dig into the code)
I was able to make it work by adding a Probe object targetting the local Talos API (just so that it works) using the meta write command:

interval: 2m
failureThreshold: 1
tcp:
  endpoint: 127.0.0.1:50000
  timeout: 1m
layer: platform

This way Talos does not check the default route but instead the probe.

Opinion

In my opinion,

• It should be handled in the install config I would see a boolean flag to disable this connectivity check (because in some air-gapped environments IMHO we may not have any relevant endpoint to hit)
• And maybe being able to set Probes from the configuration file makes sense
• I think worth mentioning in the air-gapped documentation as well

(I'm not yet super familiar with the Talos code base but, once I have a bit of time, I might be able to shoot a PR about it if it's fine for you)

Start with the air-gapped documentation and the Probe configuration described in the issue, including the meta write workaround targeting the local Talos API. Document the no-default-route situation, explain how the Probe avoids the failed connectivity check, and make the workaround easy to find for air-gapped users.