Enable Dependabot for GitHub Actions (modelcontextprotocol#1048)

Configure Dependabot to automatically detect updates to the actions used in
GitHub Actions (e.g., `actions/checkout`) and create pull requests for them.

For example, `actions/checkout@v4` is likely updatable, as v6 is already available:
https://github.com/modelcontextprotocol/inspector/blob/9f3b4ff/.github/workflows/main.yml#L15

This PR enables Dependabot to update such GitHub Actions dependencies.

Updates to outdated actions packages in `.github/workflows` are intentionally not included in this PR,
as they will be updated by the initial Dependabot run as part of verifying its behavior.

Based on experience, checking for updates on a weekly interval should work well.

Author: koic

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"