Start Creating An SDK Book

[DogeDark]

Add an mdbook for SDK for use in the docsite similar to how the current Dioxus mdbook is set up. This will probably be moved to the docsite repo once it's ready.

The book should have an introduction section with details of the project structure and layout. There should be another section called something like "Modules" that has pages on every SDK module and basic usage.

This book could include the contributor guide mentioned in #26