Setup Docs deployment

[oclero]

Setup steps

1. Create a GitHub project named qlementine-docs
2. Get the token for this project in read+write mode.
3. Get the token for qlementine in read-only mode.
4. Enable GitHub pages to serve qlementine-docs content.

CI steps

1. In qlementine project, on every commit on the master branch:
   1. Checkout master
   2. Run mkdocs to get the website built to /public folder.
2. After that, in qlementine-docs project:
   1. Wipe out qlementine-docs content.
   2. Push the content of the /public folder to qlementine-docs

[oclero]

See https://github.com/marketplace/actions/github-pages-action