Add readthedocs config

I finally got the bliss name on readthedocs, it is visible here:

• https://bliss.readthedocs.io/en/readthedocs-config

There is a webhook configured on our Gitlab to inform Readthedocs when a commit is pushed.

Readthedocs is configured to build doc for certain branches.

When it triggers a build, it clones the branch and use the .readthedocs.yaml file to know how to build the doc.