Add backup-websites script

Add a bash script that downloads and backs up my static websites from my
server onto my laptop.

nix/pkgs/backup-websites.nix

@@ -0,0 +1,13 @@
+{ lib, pkgs, ... }:
+
+pkgs.writeShellApplication {
+  name = "backup-websites";
+
+  text = ''
+    backup_path="''${HOME}/server-backup"
+
+    mkdir -p "''${backup_path}"
+
+    ${lib.getExe pkgs.rsync} -avzP ssh.oliverdavies.uk:/var/www/vhosts/ "''${backup_path}" --delete-after
+  '';
+}