Add known issue of artifactory while configuring Maven remote repo.

Details
There is a known issue in the Artifactory when you try to configure remote repo - "Whenever you test the connection you always get the "Not found" message but if you ignore that and add the repository it works flawlessly".

https://community.sonatype.com/t/maven-nexus-v3-remote-repository-integration-with-artifactory/2149.

Known issue can be added at following place in the doc:
https://docs.camunda.org/get-started/apache-maven/#camunda-nexus