[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.
I think adding instructions for self signed certificates complicates the steps for people that are doing the right thing (non-self-signed). Please keep it simple and if you add self-signed stuff (which I discourage) please do it in a separate section.