Use local playground instead of play.rust-lang.org

[C-o-d-e-C-o-w-b-o-y]

The HTTP call to https://play.rust-lang.org/evaluate.json takes very long and sometimes times out, resulting in a painful learning experience considering that you need to run it after each change.

When doing mdbook serve, we should also spin up our own local server and relay the output to the locally hosted site

[sunface]

Thanks for your suggestions, I will add some guidlines on how to use local server to provide smoother learning experience.

PRs are also welcoming :D