Clarify quickstart instructions 2.2 -> 2.3

[jackmac92]

Expected Behavior

Following the quickstart guide should produce a system which successfully boots after nixos-install --flake .#NixOS completes successfully

Current Behavior

Boot fails because it cannot find filesystem by label

Possible Solution

I think this issue should be addressed by running nixos-generate-config --root /mnt. But simply running that before nixos install doesn't work
The beginning of 2.3 says

Assuming you're happy with your existing partition layout, you can generate a basic NixOS configuration ...

Which sounds like it implies that I would run steps in 2.3 before rebooting? But I can't get bud commands to work in /iso/devos because it is readonly and not a git repository

I'm sure I'm doing something wrong, but think the quickstart could be a bit clearer here