HIG isn't structured to answer actual questions #1
Description
Right now the HIG is kind of sorted in a way where you have to read the whole thing to be able to answer design questions, which is not really practical and doesn't work well as a quick reference. The HIG would probably be a lot more useful if it was structured in a way that helps make design decisions.
For example, the "selection controls" section should be top level in the sidebar and is a good example here. It answers the question "How should I present choices to my users?". You get a quick summary of the various ways you can present choices and how to quickly decide between them, and then you get more information in each section about how to use these widgets.
In the same way, we should have sections like "Navigation" that help you decide when to use a sidebar or a notebook or a stack switcher. Or a section about providing feedback that explains when to use a dialog, infobar, toast, or notification. Welcome screens should be explained in the section about first run experience.