Building the documentation should not show any warning.

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)