[Documentation] simplify HTTPS setup details

Move all HTTPS setup details to one location and cleanup HTTPS related documentation. Previous documentation had conflicting versions of how to setup and configure various pieces. https://github.com/gitlabhq/gitlab-shell/pull/172 is related to this PR.
It is already in this guide, this is just updating to clarify that you need to use the normal or SSL version depending on your setup. Should I review the history and try to determine what version-by-version guides the updates should be added to?