Add documentation for generating bootstrap mirrors for air-gapped systems

[climbfuji]

Is your feature request related to a problem? Please describe.
Our documentation in readthedocs describes how to create source mirrors for air-gapped systems, but not how to bootstrap spack there.

Describe the solution you'd like
Add information for creating bootstrap mirrors for air-gapped systems. For now, include information on known bugs and workarounds (see spack/spack#48163).

Additional context
Keep track of spack/spack#48163 - once resolved (hopefully!), update documentation as needed