Use the `update-flake.sh` script to auto-update the NixOS version at
various places.
Also update `examples/container/flake.nix` which has been
missing in the update instructions.
Nix 2.24 is the default in NixOS 24.11.
This is required by the following commit that uses a `flake update`
command syntax only supported by Nix >= 2.19.
This checks that creating module docs for search.nixos.org succceeds.
Errors can happen when complex `default` values can't be evaluated
or on malformed Docbook XML in descriptions.
