Fixes include:
- GitLab 10.0 depends on the tracking database (because of the removal
  of System Hooks in gitlab-org/gitlab-ee!2644), therefore the docs
  should clearly state how to set up and update the Geo tracking
  database.

- To initiate synchronization it's no longer needed to push first.
  Geo will automatically pick up new projects, and it's no longer needed
  to push to the repo before GitLab Geo will start syncing.

- Small changes to improve consistency through the whole docs.

Closes gitlab-org/gitlab-ee#3365 and gitlab-org/gitlab-ee#3400