This PR is aimed at making the this repository, and the
`rustwasm.github.io` domain, the one stop shop for all documentation for
Rust and WebAssembly. Currently we have a number of sources of documentation:

* https://rustwasm.github.io/book - a tutorial and guide to Rust and WebAssembly
* https://rustwasm.github.io/wasm-bindgen - a tutorial and guide
  specific to `wasm-bindgen`
* https://rustwasm.github.io/wasm-pack - a tutorial and guide
  specific to `wasm-pack`
* https://rustwasm.github.io/ - the Rust and WebAssembly blog

This commit reorganizes all of these properties to instead be deployed
in this repository in one location, hopefully making it easier to
discover more of them and also cross-link between them. The setup here
is somewhat similar to the Rust documentation itself, comprising of a
number of books.

When this site is built it will clone the book, wasm-bindgen, and
wasm-pack repositories. The latter two will be reset to their most
recent tag, matching the most recently published version on crates.io.
All books are then compiled with `mdbook` and placed into the output here.

The current set is then laid out as follows:

* https://rustwasm.github.io/ - landing page, button to install
  wasm-pack, links everywhere else
* https://rustwasm.github.io/blog.html - new home for the blog
* https://rustwasm.github.io/docs.html - dispatch point for documentation

I'm hoping that we can centralize here by using
https://rustwasm.github.io if anyone wants an entry point or an
easy-click installer and using `/docs.html` as the main location for
documentation.

Once this lands I'd like to update the deployments of the book,
wasm-bindgen, and wasm-pack. All those books will gain a banner saying
they're a "nightly preview" with links to the officially published
versions. This means that visiting https://rustwasm.github.io/book will
have a banner saying you should go to
https://rustwasm.github.io/docs/book instead basically. The landing page
at https://rustwasm.github.io/wasm-pack will probably stay for now (and
the installer page will definitely stay), but we likely won't link to it
much any more.

Finally for a deployment strategy I hope to turn this into a daily cron
job for this repository. That means we'll redeploy everything at least
once a day, picking up any changes naturally. If we want to manually
trigger a redeploy, however, we can simply reexecute the previous build
on Travis.