Add Dependabot configuration for GitHub Actions updates (#1217)

Co-authored-by: DarkLight1337 <tlleungac@connect.ust.hk>

.github/dependabot.yml

+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"