Update docs site guide for Airgap

General MR

Summary

The current documented airgap process is using a very manual process and a deprecated tool with code that was last touched 3+ years ago.

I would recommend switching the docs site to display the airgap with zarf guide since that is using a current tool and appears to be edited more recently by the BB dev team. The BB pipeline for airgap also uses zarf so this seems to make a lot of sense.

Note that this doesn't delete any content - just swaps what is more prominently displayed on the docs site so that people don't get lead to an outdated process.