ci: add dependabot npm ecosystem for docs/user dependencies

Configure Dependabot to monitor and update npm dependencies in the
docs/user directory (VitePress documentation). Follows same monthly
schedule and PR limits as Go modules and GitHub Actions.

Author: orien

.github/dependabot.yml

@@ -33,3 +33,13 @@ updates:
       prefix: "ci"
       prefix-development: "ci"
       include: "scope"
+
+  - package-ecosystem: "npm"
+    directory: "/docs/user"
+    schedule:
+      interval: "monthly"
+    open-pull-requests-limit: 20
+    commit-message:
+      prefix: "docs"
+      prefix-development: "docs"
+      include: "scope"