Fix charts/gitlab#654 by updating nginx ingress documentation

The command line options documentation contains a list of chart's values except for the nginx ingress. I assume this was intended because of the amount of values for the nginx ingress documentation and/or because the nginx ingress chart is synced with the upstream chart so the values may change.

The upstream chart specifies values as controller.image.tag, but it really should be specified as nginx-ingress.controller.image.tag. I added documentation to add the nginx-ingress prefix.

In addition, the link in doc/charts/nginx/index.md linked to itself. I assumed it was meant to link to charts/nginx/index.md. I updated this link.

Fixes charts/gitlab#654