Merge pull request #8788 from reitermarkus/dependabot

Add `dependabot.yml`.

.github/dependabot.yml

+version: 2
+
+updates:
+
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+
+  - package-ecosystem: bundler
+    directory: /docs
+    schedule:
+      interval: weekly
+
+  - package-ecosystem: bundler
+    directory: /Library/Homebrew
+    schedule:
+      interval: weekly