add dependabot configuration

.github/dependabot.yml

@@ -0,0 +1,40 @@
+version: 2
+updates:
+  - package-ecosystem: "bundler"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+      timezone: "Europe/Helsinki"
+    groups:
+      bundler:
+        patterns:
+          - "*"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+      timezone: "Europe/Helsinki"
+    groups:
+      github-actions:
+        patterns:
+          - "*"
+
+  - package-ecosystem: "gitsubmodule"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+      timezone: "Europe/Helsinki"
+    groups:
+      gitsubmodule:
+        patterns:
+          - "*"
+
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    groups:
+      npm:
+        patterns:
+          - "*"