"Package" option allows specifying the derivation that will be used for
the systemd service. Works the same way as `services.<name>.package` in
NixOS. By default picks the docspell packages from pkgs instance -- same
behavior as prior to this commit
Closes#2627
This commit adds a new option for joex and server modules - extraConfig.
Its main use-case is to specify options that are present in docspell but
not (perhaps, yet) defined in the modules.
This commit introduces the ability to install Docspell from a Nix flake.
There are no major changes to the logic of the previous modules outside of
organizing them in a flake and adding a simple check script.
