Updates and modifications for ease of use on Linux

[qdzlug]

This change adds in a number of changes to help customers more easily build images for the Oxide Rack.

Key Changes:

• New documentation focusing on the Linux use case.
• "imgbuild.sh" script and associated modules to provide a simple way to perform necessary tasks to get from iso -> image.
• Modification of internet connectivity check from ping (which does not work under qemu's default networking) to a check of port 443 on oxide.computer.

[wfchandler]

With the caveat that I haven't tested this, it looks good. I think we could replace build_app.sh with a simple cargo run, but don't feel strongly about it.

[wfchandler]

We could use cargo run to execute the binary and build it if necessary, that would let us skip the build_app.sh script.

[qdzlug]

I'm trying to remember why I did that - there was a reason, but I have nfi what that is now. I'm going to merge this in and then revisit with my next round of changes (there are a few open PR's that I want to rebase onto this), and I'll just modify it then (once I figure out what I'm doing).