Add Dependabot configuration for automated dependency updates (#22)

Configure Dependabot to automatically check for updates to GitHub Actions
and Cargo dependencies on a weekly schedule. This will help maintain
security and keep dependencies current by creating pull requests for
available updates.

The configuration enables:
- GitHub Actions dependency updates from the root directory
- Rust/Cargo dependency updates from the root directory
- Weekly update schedule for both ecosystems

This automation reduces maintenance burden and improves security posture
by ensuring timely dependency updates.

Author: linw1995

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"