Bump Ruby gems automatically with Dependabot for docs (#9401)

Previous Ruby gem version bumps were only triggered for security updates (since Dependabot was left unconfigured for bundler).

See: https://github.com/FreshRSS/FreshRSS/pulls?q=is%3Apr%20state%3Aclosed%20label%3Aruby%20author%3Aapp%2Fdependabot

.github/dependabot.yml

@@ -35,3 +35,9 @@ updates:
       interval: "monthly"
     cooldown:
       default-days: 7
+  - package-ecosystem: "bundler"
+    directory: "/docs/"
+    schedule:
+      interval: "monthly"
+    cooldown:
+      default-days: 7