[DOC] add demo/example config for subvolumes on btrfs on lvm on luks

[nickgarber]

Many helpful examples have been added over the past year or so but I haven't seen this one.

There are existing examples of each of these components separately, and in various useful combinations, but not quite in this particular combo.

I acknowledge it would be possible to mentally synthesize the existing examples into this requested config, but there a lot of clarifying value in an example to demonstrate that an approach is viable, and to serve as a lighted path.

This config is useful for folks who're interested in using/exploring btrfs in a secure configuration but also value retaining the ability to carve out other filesystems where it's not the ideal match, (such as with DB workloads).

[Mic92]

Feel free to add more real-world examples in https://github.com/nix-community/disko-templates
If you struggle to come up with the configuration, @Lassulus can help you in the disko matrix channel at https://matrix.to/#/#disko:nixos.org