Alp Mestanogullari authored May 09, 2019 and Ben Gamari committed May 14, 2019

Previously, changing one of the .rst files from the user guide would not cause
the user guide to be rebuilt. This patch take a first stab at declaring the
documentation source files that our documentation rules depend on, focusing
on the .rst files only for now.

We eventually might want to rebuild docs when we, say, change the haddock style
file, but this level of tracking isn't really necessary for now.

This fixes #16645.