Add gitlab pages with the docs

This MR is to generate the documentation of the project through CI/CD.

It adds a new job in the .gitlab-ci.yml file that build the docs and move them to a gitlab page. In that way we can have our documentation hosted by gitlab.

It also modifies a bit the README.md file in order to match with the latest changes.

The last modified file is the conf.py for the sphinx documentation, that has been modified in order to mock the modules that are not possible/needed to import during the documentation building.