DOC-3234: re-indroduce deleted nav link.

Author: kemister85

modules/ROOT/nav.adoc

@@ -53,6 +53,7 @@
 **** xref:dotnet-projects.adoc[.NET projects]
 **** xref:bower-projects.adoc[Bower projects]
 **** xref:zip-install.adoc[{productname} .zip deployments]
+*** xref:upgrading.adoc[General upgrade information]
 * xref:how-to-guides.adoc[How-to guides]
 ** Learn the basics
 *** xref:basic-setup.adoc[Basic setup]