Rework docs

[davidpdrsn]

This reworks axum's docs in an attempt to make things easier to find. Previously I wasn't a fan of those docs for the same topic were spread across the root module docs and more specific places like types and methods.

This changes it such that the root module docs only gives a high level introduction to a topic, perhaps with a small example, and then link to other places where all the details are. This means Router is now the single place to learn about routing, and etc for the topics like handlers and error handling.