[Security] Adds placeholder page for privileged user monitoring (#1815)

Adds a hidden placeholder page for privileged user monitoring to enable
linking to the future doc from the product onboarding page.

Author: natasha-moore-elastic

solutions/security/advanced-entity-analytics/privileged-user-monitoring.md

@@ -0,0 +1,13 @@
+---
+applies_to:
+  stack: preview 9.1
+  serverless:
+    security: preview 9.1
+products:
+  - id: security
+  - id: cloud-serverless
+---
+
+# Privileged user monitoring
+
+This page is a placeholder for future documentation.

solutions/toc.yml

@@ -657,5 +657,6 @@ toc:
               - file: security/advanced-entity-analytics/anomaly-detection.md
               - file: security/advanced-entity-analytics/optimizing-anomaly-results.md
               - file: security/advanced-entity-analytics/behavioral-detection-use-cases.md
+          - hidden: security/advanced-entity-analytics/privileged-user-monitoring.md
       - file: security/asset-management.md
       - file: security/apis.md