Consolidate Security Guard Content (#6464)

* Consolidate Security Guard Content

* fix titling and mention maturity level

* drop unnecessary links

Author: dprotaso

docs/versioned/.nav.yml

@@ -102,10 +102,7 @@ nav:
           - Configuring Activator capacity: serving/load-balancing/activator-capacity.md
       # Serving - Application Security
       - Application security:
-        - About Security-Guard: serving/app-security/security-guard-about.md
-        - Installing Security-Guard: serving/app-security/security-guard-install.md
-        - Security-guard quickstart: serving/app-security/security-guard-quickstart.md
-        - Security-Guard example alerts: serving/app-security/security-guard-example-alerts.md
+        - Security Guard: serving/app-security/security-guard.md
       # Serving - observability
       - Observability:
         - Collecting metrics: serving/observability/metrics/collecting-metrics.md