docs: in ref relation add link to manage relations

Author: tmihoc

reference/relation.md

@@ -1,5 +1,6 @@
 (relation)=
 # Relation
+> See also: {ref}`manage-relations`
 
 In JAAS, like in [OpenFGA](https://openfga.dev/docs/concepts#what-is-a-relation) and unlike in{external+juju:ref}`Juju <relation>`, a relation is about Relation Based Access Control (**ReBAC**) and describes the relation between two entities.