WIP: build docs on CI and deploy to GitHub pages

[timvisee]

Note: this PR is not complete, it requires some changes by @houqp

Fixes #5

Here I've attempted to start on automatically building docs and deploying it to GitHub pages. I've used the following article as a reference from the CircleCI blog: https://circleci.com/blog/deploying-documentation-to-github-pages-with-continuous-integration/

I've added a step for documentation deployment. The docs being built at target/doc/ are persisted, and used for deployment. Docs are only deployed when pushed on the master branch.

I believe it's mostly complete, but it requires the configuration of a key to allow pushing back to the repository on the gh-pages branch.

@houqp To complete this, you'll just have to look at the last two sections (from Provisioning a Deploy Key): https://circleci.com/blog/deploying-documentation-to-github-pages-with-continuous-integration/#provisioning-a-deploy-key (scroll up a little, the anchor is covered by the header)

I can't properly test this, sadly. But the implementation looks al right. You can continue on my branch with these last few changes if you desire before merging the request.

[timvisee]

Just cleaned up my branch log to remove some merge commits, the end result is the same.

[houqp]

Thanks! I am working on setting up the deploy key now.