CI: Introduce website stage (#433)

trigger job only on default branch

`htmlcov` is necessary at the root of the repository
to build the website with the coverage in

Push on `pages` branch with an access token
set in CI/CD variables

The access token has to be renew every year,
since it’s the maximum lifespan of an access token

Access token has to be created on a project basis with
`repository_write` checked and Developer role

https://www.benjaminrancourt.ca/how-to-push-to-a-git-repository-from-a-gitlab-ci-pipeline/
https://stackoverflow.com/questions/51716044
https://stackoverflow.com/questions/25409700
https://docs.gitlab.com/ee/user/project/settings/project_access_tokens.html