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/flake.lock

@@ -0,0 +1,26 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1669735802,
+        "narHash": "sha256-qtG/o/i5ZWZLmXw108N2aPiVsxOcidpHJYNkT45ry9Q=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "731cc710aeebecbf45a258e977e8b68350549522",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-22.11",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}