Improve maintainability of navigation.

This is an attempt to create an authoritative place to describe expected
site page flow.

- Add make_nav script which programmatically generates ddh-index.md and
  ddh-prev-next.json to keep them synced.
- Align page titles and header displays.
- Include the files so generated.

The script is intended to be run by hand when changing page titles
or the navigation structure.

The script includes some assumptions about repository layout which can
be adjusted as needs change.