Configure dependabot to handle rubygems dependencies (#9445)

Merge pull request 9445

.github/dependabot.yml

@@ -4,3 +4,9 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+  - package-ecosystem: bundler
+    reviewers:
+      - "jekyll/core"
+    directory: /
+    schedule:
+      interval: weekly