Since the release of sphinx 4.1, a new warning related to httpdomain
extension appeared when buidling swh-web documentation.
It is not critical and is due to httpdomain not in sync with latest
changes in sphinx, so filter out that warning as it makes sphinx
builds fail for swh-web on Jenkins.
A pull request has been created to fix the issue,
the introduced hack should be removed if it gets merged and a new
sphinx-httpdomain version released.