Make the book accessible through GitHub Pages

[KodrAus]

Our initial CI script in copied from the rustc dev guide, but we don't currently have a gh-pages branch, so will probably need to do some more work to get it readable.

[simonsan]

Can do that :)

[simonsan]

Used our setup from https://github.com/rust-unofficial/patterns/ though