MNT: Add Dependabot config for GitHub Actions updates

* Add Dependabot config for weekly updates to GitHub Actions for
  security best practices.
   - c.f. https://learn.scientific-python.org/development/guides/gha-basic/#updating

Author: matthewfeickert

.github/dependabot.yml

@@ -0,0 +1,12 @@
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    labels:
+      - "github-actions"
+      - "dependencies"
+    reviewers:
+      - "asmeurer"