Use data feature to define docs index

@mattr- got me thinking about ways to sort pages in #1941, specifically around documentation, and I think this is a pretty cool way to do it. It further simplifies the awesome table of contents stuff that @penibelst did via 3f7dcaa2ec7fea5985e6986cde7ec170d398adc4, but eliminates the double content that unfortunately still existed there.

Change the data file, change the table of contents.