Automatically build and deploy documentation

[mtazzari]

Following this tutorial:
https://circleci.com/blog/automate-your-nuxt-js-app-deployment/

Note: CircleCI only accepts SSH private keys in PEM format, but it is not well documented.
This is useful: https://discuss.circleci.com/t/adding-ssh-keys-fails/7747/24
So, the solution is to use:

 ssh-keygen -m PEM -t rsa -b 4096 -C "ci-build@ast.cam.ac.uk"

instead of

 ssh-keygen -t rsa -b 4096 -C "ci-build@ast.cam.ac.uk"

which is suggested in the tutorial.

[mtazzari]

Documentation:

• is built automatically for each push to any branch;
• is deployed to gh-pages for each push to master.

This PR Fixes #4

I merge to master so that the first draft of the documentation is deployed to gh-pages and we check the rendering.