[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.
https://github.com/gitlabhq/gitlabhq/pull/7583/files#diff-de9999f853c82dee91389875a9737878R22 and https://github.com/gitlabhq/gitlabhq/pull/7583/files#diff-c462b872cf0b6afb467f894b4b1cf90fR347.