Docs: Pages - clean up articles
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/39978
- Cleans up the "GitLab from A to Z" series
- Update tutorials (remove the Shared Runners step, update project setting navigation)
EE-version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4599
Edited by Marcia Ramos