docs: refactor

docs/manual/custom-plugins/old-method.md

@@ -0,0 +1,18 @@
+# Old Method {#sec-old-method}
+
+Users who have not yet updated to 0.5, or prefer a more hands-on approach may use the old method where the load order
+of the plugins is determined by DAGs.
+
+```nix
+{
+  # fetch plugin source from GitHub and add it to startPlugins
+  config.vim.startPlugins = [
+    (pkgs.fetchFromGitHub {
+      owner = "FrenzyExists";
+      repo = "aquarium-vim";
+      rev = "d09b1feda1148797aa5ff0dbca8d8e3256d028d5";
+      sha256 = "CtyEhCcGxxok6xFQ09feWpdEBIYHH+GIFVOaNZx10Bs=";
+    })
+  ];
+}
+```