docs: update reference docs table of content (#1113)

Author: lukasmetzner

docs/reference/README.md

@@ -5,3 +5,4 @@ In this folder, you should find technical references material of the hcloud-clou
 - [Helm](helm/README.md)
 - [Version Policy](version-policy.md)
 - [Load Balancer Annotations](load_balancer_annotations.md)
+- [Load Balancer Environment Variables](load_balancer_envs.md)