Support five levels of nested items in the global nav

Right now, we may nest up to 3 items in the global nav. We have 3 items (top to bottom):

- section_title
- category_title
- doc_title

We should make this more generic, so that we can nest as many items as we want (five being a good amount).

For example, Docker has generic items that they can nest indefinitely:

- sectiontitle:
  section:
  - path:
    title:
  - sectiontitle:
    section:
    - title:
      path:
    - sectiontitle:
      section:
      - sectiontitle:
        section:
        - path:
          title: