Conditionalize note about outbox event router incompatibility

In outbox-event-router.adoc, the note about the SMT not being supported with MongoDB is currently not conditionalized. As a result, the content is rendered in the downstream documentation. This is confusing, because the text of the note instructs users to "please use the MongoDB outbox event router SMT", and includes a link to the documentation for that SMT. However, neither the SMT or the targeted documentation are available downstream. And because the link targets an unavailable file, it causes the downstream documentation build to fail.
Update the note to conditionalize the sentence that suggests use of the alternative SMT as community content as follows:

[NOTE]
====
The outbox event router SMT is not compatible with the MongoDB connector.

ifdef::community[]
As an alternative, MongoDB users can run the xref:transformations/mongodb-outbox-event-router.adoc[MongoDB outbox event router SMT].
endif::community[]
====