fix: add celery worker / beat to nav.adoc (#767)

Author: maltesander

docs/modules/superset/partials/nav.adoc

@@ -5,6 +5,7 @@
 * xref:superset:usage-guide/index.adoc[]
 ** xref:superset:usage-guide/listenerclass.adoc[]
 ** xref:superset:usage-guide/storage-resource-configuration.adoc[]
+** xref:superset:usage-guide/celery-async-queries.adoc[]
 ** xref:superset:usage-guide/database-connections.adoc[]
 ** xref:superset:usage-guide/security.adoc[]
 ** xref:superset:usage-guide/connecting-druid.adoc[]