boot.initrd.luks.devices: Change into an attribute set

This allows setting options for the same LUKS device in different
modules. For example, the auto-generated hardware-configuration.nix
can contain

  boot.initrd.luks.devices.crypted.device = "/dev/disk/...";

while configuration.nix can add

  boot.initrd.luks.devices.crypted.allowDiscards = true;

Also updated the examples/docs to use /disk/disk/by-uuid instead of
/dev/sda, since we shouldn't promote the use of the latter.