As they are shown among quite a lot of output, it's hard to see when we introduce a new warning, that may be more important than the other ones.
Therefore, we should use the `-W` option of `sphinx-build`, to crash whenever there is a warning.
But first, we need to fix all current warnings that show up when building the doc (`make -C docs clean && tox -e sphinx-dev -r` in swh-docs)