Add Exception Safety topic to User Guide (#224)

Author: PeterTurcan

user-guide/modules/ROOT/nav.adoc

@@ -28,6 +28,9 @@ Official repository: https://github.com/boostorg/website-v2-docs
 ** xref:task-natural-language-parsing.adoc[]
 ** xref:task-parallel-computation.adoc[]
 
+* Development
+** xref:exception-safety.adoc[]
+
 * Community
 ** xref:reporting-issues.adoc[]
 ** xref:discussion-policy.adoc[]