Follow-up from "Update left menu for Omnibus"

The following discussion from !699 (merged) should be addressed:

• @eread started a discussion:

        - category_title: Prepare a build environment
          category_url: 'build/README.html'
          docs:
            - doc_title: With Docker
              doc_url: 'build/prepare-build-environment.html'
        - category_title: Set up a development environment
          category_url: 'development/setup.html'
        - category_title: Config options
          category_url: 'settings/configuration.html'
        - category_title: Changing YAML config options
          category_url: 'settings/gitlab.yml.html'

  This hierarchy makes more sense to me. We can look at it in a follow up.