Convert first chapter to mdbook

Now that mdbook can support the Ace editor (thanks @projektir!),
we can start moving RBE over to mdbook.

This removes everything but the content, sets up book.toml, and
converts the first chapter.

https://github.com/azerupi/mdBook/pull/338

Author: steveklabnik

.gitignore

@@ -1,7 +1 @@
-bin/*
-stage/*
-node_modules/*
-!node_modules/gitbook-plugin-rust-playpen
-node_modules/gitbook-plugin-rust-playpen/book/mode-rust.js
-node_modules/gitbook-plugin-rust-playpen/book/ace
-target
+book