use h1 tags to not render titles in sidebar (#626)

docs/reference/cluster_manifest.md

@@ -1,4 +1,4 @@
-# Cluster manifest reference
+<h1>Cluster manifest reference</h1>
 
 Individual Postgres clusters are described by the Kubernetes *cluster manifest*
 that has the structure defined by the `postgresql` CRD (custom resource

docs/reference/command_line_and_environment.md

@@ -28,7 +28,7 @@ flags](https://godoc.org/github.com/golang/glog) are also supported. For
 instance, one may want to add `-alsologtostderr` and `-v=8` to debug the
 operator REST calls.
 
-## Environment variables
+# Environment variables
 
 The following environment variables are accepted by the operator:
 

docs/reference/operator_parameters.md

@@ -1,4 +1,4 @@
-# Configuration parameters
+<h1>Configuration parameters</h1>
 
 There are two mutually-exclusive methods to set the Postgres Operator
 configuration.