Update index.md to ellaborate on port configuration and accessing Prometheus from non-localhost hosts.

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Previously, the documentation did not (to my knowledge) mention that Prometheus was designed "to only listen on localhost by default", and has since been updated to allow for non-localhost hosts to access it. The documentation now accounts for this and describes the configuration required to allow for external access to the Prometheus service.