Link from the values.yaml documentation to the respective detailed docs

We have a single list of all of our configuration options, which is great: https://gitlab.com/charts/gitlab/blob/master/doc/installation/command-line-options.md

We should enhance this list, by adding links for each configuration item to the respective documentation. This would make it much easier to use.

We should also make sure we do the same within values.yaml itself, similar to what we do with gitlab.rb. (We have done this in some cases, but we should do it for all.)