chore: Expand Dependabot coverage (#1389)

This PR configures Dependabot to check for updates to the root
`package.json`, the new ‘Auth’ sub-action’s `package.json`, and the demo
Jekyll site’s `Gemfile`.

Author: smockle

.github/dependabot.yml

@@ -3,6 +3,14 @@
 
 version: 2
 updates:
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+  - package-ecosystem: "npm"
+    directory: "/.github/actions/auth"
+    schedule:
+      interval: "weekly"
   - package-ecosystem: "npm"
     directory: "/.github/actions/find"
     schedule:
@@ -35,3 +43,7 @@ updates:
     directory: "/.github/actions/gh-cache/save"
     schedule:
       interval: "weekly"
+  - package-ecosystem: "bundler"
+    directory: "/sites/site-with-errors"
+    schedule:
+      interval: "weekly"