Include docs building and deployment into the CI/CD pipeline

With this MR, the building and deployment of the documentation are added to the CI/CD configuration. In there, the pip install -e .[docs] and make html commands are used to build the documentation locally. Then, the artefacts are moved to the public folder in order to deploy the documentation to GitLab pages. Currently, the docs are build only for the main branch and the respective pipeline is triggered for each commit to main. Building and deployment of both versions stable (i.e. from the main branch) and latest (i.e. from the dev branch) will be added in a separate MR.