ci: add dependabot yaml (#13)

* ci: add dependabot yaml

* ci: add cooldown to Dependabot action updates

Wait 3 days after a release before proposing it, so a freshly published
(and potentially compromised) action version ages out in the open before
landing in a PR. Applies to version updates only; security advisories
still bypass the cooldown.

Author: guru-desh

.github/dependabot.yml

@@ -0,0 +1,14 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 3
+    commit-message:
+      prefix: "ci"
+    groups:
+      actions:
+        patterns:
+          - "*"