[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.
See https://github.com/gitlabhq/gitlabhq/pull/7583/files#diff-c462b872cf0b6afb467f894b4b1cf90fR345. I believe it is easier to centralize all the necessary changes in `installation.md`.