Docs: update GitLab products description
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
- For https://gitlab.com/gitlab-org/gitlab-ce/issues/42631 and gitlab-com/www-gitlab-com#1906 (closed)
- Based on gitlab-com/www-gitlab-com!9735 (merged)
- Updates the description for CE, EES, EEP, EEU to Libre, Starter, Premium, Ultimate.
- Adds description for GitLab.com plans
Edited by Marcia Ramos