Book structure

[alice-i-cecile]

I've created the skeleton of the new book following bevyengine/rfcs#23, and added brief notes to each chapter on what I think should be covered and in what order.

Merging this in will allow us to start writing in a nice, distributed fashion. If you want to tackle a chapter, please mention it in #158 so we don't duplicate effort. I would particularly appreciate help with the platform-specific chapters, rendering and assets, as those are the areas I know least about.

There were two areas of content that were lost from the old book:

1. Contributing: moved to [Merged by Bors] - Comprehensive CONTRIBUTING.md bevy#2040
2. Migration guides: I strongly feel that this is a better fit to link at the top of each release blog post.

[alice-i-cecile]

As you can see by the ridiculous screen shots, #153 is needed to make this expanded structure at all sensible to navigate 😆

[cart]

Solve the "mouse" thing and I think this is good to merge!

[alice-i-cecile]

This is good to merge now @cart, likely along with #137 :)