Start adding kernel docs to the book

[bradjc]

This will make the book the go-to place for documentation and help with Tock.

We don't need to move everything from the tock/doc folder. But most of these documents don't change very often.

Also, the book is just a better format to view the docs in than readme files in the github repo.

[hudson-ayers]

I went to look at the rendered pages, but the build failed -- I think you need to run make pretty to fix the formatting issues