[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.
Unless you can provide a case where no certificate would be better than a self-signed certificate I prefer we keep some directions about it. I have reworded it in https://github.com/bbodenmiller/gitlabhq/commit/61f903f6eaf8d3b82ca421ca87bf1800a015f9cc.