nix: implement flake

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.

nix/checks/default.nix

@@ -0,0 +1,7 @@
+{ ... }:
+{
+  imports = [
+    ./configuration-test.nix
+    ./solr.nix
+  ];
+}