nav: sub-page gets skipped

Steps to reproduce:

1. Navigate to https://dvc.org/doc/use-cases/data-registry.
2. Scroll to the bottom and click "Next."

I think it's a bug related to it being last in the list of use cases, so one option is to move it up in the order, which seems reasonable.

Reproduce the issue at the linked data-registry page by clicking “Next” after scrolling to the bottom. Trace the Gatsby theme’s use-case navigation and verify that the sub-page is included in the order; done means the Next link reaches the intended following page without skipping it.

Written by the indexing model from the issue text.