Move docs to docs directory

[honzabrecka]

https://help.github.com/articles/configuring-a-publishing-source-for-github-pages/#publishing-your-github-pages-site-from-a-docs-folder-on-your-master-branch

Main purpose is to clean up the root directory and remove the need for the manual sync between master/gh-pages branches.

It also introduces Dockerfile, so potential contributors do not need to install ruby etc. stuff just to build/watch docs.

cd docs
docker build -t wu-docs .
docker run --rm -p 4000:4000 -v "$PWD:/usr/src/app" wu-docs