Docs: add EEU tier to the landing page
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
- Adds EEU to the list of EE tiers
- Un-indexes technical articles from the landing page (link to Contributor docs instead)
cc/ @axil @williamchia FYI
Edited by Marcia Ramos