Documentation: Indexes per Topic

What

Create indexes per topic to gather all the content available through GitLab on specific subjects

Folder structure

Proposal

- doc/
  - topics/
    - index.html
    - pages/
    - gitlab-ci/
    - idea-to-production/
    - etc

Existing models

• GitLab Pages - https://docs.gitlab.com/ce/user/project/pages/index.html
• GitLab CI (mockup) - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9861

cc/ @axil @seanpackham