[REQUEST]: Add note to air-gapped docs for new isAirGapped flag

[kpollich]

Description

We added a new xpack.fleet.isAirGapped flag in elastic/kibana#174214 that helps Fleet know when it can skip certain requests or change network behaviors in air-gapped environments. This results in a better user experience overall in air-gapped environments, and can be an escape hatch for certain issues like elastic/kibana#173125.

I'd like to add a quick note to https://www.elastic.co/guide/en/fleet/current/air-gapped.html that explains the flag, e.g.

Enable air-gapped mode for Fleet

Set the following property in Kibana to enable air-gapped mode in Fleet. This allows Fleet to intelligently skip certain requests or operations that shouldn't be attempted in air-gapped environments.

xpack.fleet.isAirGapped: true

Resources

elastic/kibana#174214

Collaboration

The product team will provide the initial content.

Point of contact.

Main contact: @kpollich

Stakeholders: @elastic/fleet