CI pipeline to build docs

[acarapetis]

This adds a workflow to circleCI triggered off the master branch which:

• Builds the sphinx documentation to HTML
• Pushes the built documentation to an orphan branch gh-pages. (at the moment using a "User key" I generated via the circleCI config that grants it my permissions - feel free to change this)

Hopefully you should be able to click a few buttons in github settings for this repo (or your account?) and we'll have auto-updated docs somewhere under github.io. :)