Create nested `datalist` with `select` fallback for part/chapter/article subheads

[reubenlillie]

Unlike the main navigation menu which mimics the printed table of contents, this feature should account for all section headings within the Manual.

This feature should be something similar to, but an improvement upon, the “Quick Reference” created by the third-party plugin on the WordPress site: http://2017.manual.nazarene.org/