Migrate Counted Body topic to User Guide (#226)

Author: PeterTurcan

user-guide/modules/ROOT/nav.adoc

@@ -31,6 +31,7 @@ Official repository: https://github.com/boostorg/website-v2-docs
 * Development
 ** xref:generic-programming.adoc[]
 ** xref:exception-safety.adoc[]
+** xref:counted-body.adoc[]
 
 * Community
 ** xref:reporting-issues.adoc[]