Please update Jenkins docs to reflect new recommended process for agents

The Jenkins/OCP team is no longer going to provide new 'worker' (agent) images.  Instead, we recommend the user can make their own, based on the blog article at [1].

Please integrate this information into the OCP docs, preferably in the sections at [2][3].

Thank you!

[1] https://developers.redhat.com/blog/2020/06/04/an-easier-way-to-create-custom-jenkins-containers

[2] https://docs.openshift.com/container-platform/4.8/openshift_images/using_images/images-other-jenkins.html

[3] https://docs.openshift.com/container-platform/4.8/openshift_images/using_images/images-other-jenkins-agent.html