Create headings for different types of reference architectures

doc/administration/reference_architectures/index.md

@@ -59,7 +59,9 @@ provided architecture.
 
 ## Available reference architectures
 
-The following reference architectures are available:
+The following reference architectures are available.
+
+### GitLab package (Omnibus)
 
 - [Up to 1,000 users](1k_users.md)
 - [Up to 2,000 users](2k_users.md)
@@ -69,6 +71,8 @@ The following reference architectures are available:
 - [Up to 25,000 users](25k_users.md)
 - [Up to 50,000 users](50k_users.md)
 
+### Cloud native hybrid
+
 The following Cloud Native Hybrid reference architectures, where select recommended components can be run in Kubernetes, are available:
 
 - [Up to 2,000 users](2k_users.md#cloud-native-hybrid-reference-architecture-with-helm-charts-alternative)