Better specify where and how to look for RHEL packages

When submitting new dependencies to base.yaml, users should look for RHEL packages. https://github.com/ros/rosdistro/blob/50bf831c334b42caac3378075956c3427d2da929/.github/pull_request_template.md?plain=1#L48 However, as RHEL is not open to public, it is difficult to search for its packages. The line in PR template suggests rhel.pkgs.org, but the results there are still hard to parse. As someone who never worked with RHEL myself, I don't understand all the nuances between RHEL, Enterprise Linux (as it's called on pkgs.org), AlmaLinux, CentOS, Rocky Linux and Fedora. I think it would be great if the PR template could be a little more descriptive in what to look for. Also, it might be worth specifying which package streams are allowed in rosdistro.