chore: Add ruby to dependabot file (#263)

This is just because I see a ton of things in this tab:
https://github.com/IntelliTect/CodingGuidelines/security/dependabotf and
it makes me think maybe we just want to keep the ruby deps up to date if
possible.

Thoughts?

.github/dependabot.yml

@@ -9,3 +9,8 @@ updates:
     directory: "/" # Location of package manifests
     schedule:
       interval: "weekly"
+
+  - package-ecosystem: "bundler"
+    directory: "/docs"
+    schedule:
+      interval: "weekly"