Improve instructions for Enterprise Server customers

[hectorsector]

The existing instructions for publishing a course use a forking workflow, so it'd really only apply to Enterprise Cloud customers and general course authors. We should include additional instructions for Enterprise Server customers that include how to clone and push the repos to their GHE urls.

[brianamarie]

As a part of this, some customers will not have access to suggested changes. The instructions were a bit tricky to follow since they were formatted specifically for suggested changes. +1 that revisiting the instructions is important for the GHES ship. 👍