feat: add dependabot

Fixes #1118.
Adds `dependabot.yml` to `.github/` to check for `"github-action"`
updates on a `"weekly"` basis.
This does not touch Rust code or Cargo workflows.

It will  create PRs and we would need to approve them
(they would be subject to the same merge policy)
to instantiate the proposed dependabots into `master`.

.github/dependabot.yml

@@ -0,0 +1,8 @@
+# Set update schedule for GitHub Actions
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      # Check for updates to GitHub Actions every week
+      interval: "weekly"