Document Cypher features that are not in GQL (#1033)

Author: JPryce-Aklundh

modules/ROOT/content-nav.adoc

@@ -130,6 +130,7 @@
 *** xref:appendix/gql-conformance/unsupported-mandatory.adoc[]
 *** xref:appendix/gql-conformance/supported-optional.adoc[]
 *** xref:appendix/gql-conformance/analogous-cypher.adoc[]
+*** xref:appendix/gql-conformance/additional-cypher.adoc[]
 ** xref:appendix/tutorials/index.adoc[]
 *** xref:appendix/tutorials/basic-query-tuning.adoc[]
 *** xref:appendix/tutorials/advanced-query-tuning.adoc[]