Provide "how to use" for each section of rustc-dev-guide

[workingjubilee]

Any resource of any considerable length or value inevitably hits this point of recursion: it must instruct the user on how to use it. Thanks to the excellent work of the rustc-dev-guide contributors, the guide has not only hit this point of being so value it has more information than can simply be skimmed, it has lapped it. Now each section has plenty of information! ...enough that it, alas, requires its own organizational overhead, such as an introductory preview and sub-Table-of-Contents-like arrangement.

Content may need to be rearranged to make this particularly useful,
e.g. organizing the Linux, Mac, and Windows sections of "Building and debugging rustc".
"There isn't a distinct Linux and Mac section of 'Building and debugging rustc' tho'? Just the start and then Windows."
Exactly.