Hadrian: 'need' source files for various docs in Rules.Documentation

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 (closed).