Geo documentation fixes following on from 2017-11-14 demo

What does this MR do?

Reworks Geo documentation to streamline and improve the systems administrator experience

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Points raised during 2017-11-14 demo: https://docs.google.com/document/d/18vGk6dQs7L0oGQOb_bNiFa5JhwLq5WBS7oNxQy09ml8/edit

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
• Conform by the merge request performance guides
• Conform by the style guides
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #4052 (closed)