Add Dependabot config (#49)

To update the Ruby + GHA deps.

https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file

.github/dependabot.yml

@@ -0,0 +1,15 @@
+version: 2
+updates:
+  - package-ecosystem: "bundler"
+    directory: "/.rundoc-workspace"
+    schedule:
+      interval: "monthly"
+    groups:
+      ruby-dependencies:
+        update-types:
+          - "minor"
+          - "patch"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"