Add warning about EPEL not being supported in the repo setup section of install/update guides

[nixfu]

I think a warning about EPEL not being supported should be included in the repository setup section of the install and upgrade guides for Katello.

An update to an EPEL package recently broke pulpcore in Foreman 3.9/Katello 4.11 if you had EPEL enabled.

Right now the only little note about EPEL not being supported is pretty small in section "1.4. Supported operating systems".

A VERY logical place to have a clear notice about ensuring EPEL is not enabled, would be in part of the docs that discuss setting up the repos needed on the system for installation(Section 3)/upgrade(Section 2).