Since the release of `sphinx 4.1`, a new warning related to `httpdomain`
extension appeared when buidling `swh-web` documentation.
See https://jenkins.softwareheritage.org/job/DDOC/job/dev/6169/artifact/swh-docs/docs/errors.log
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](https://github.com/sphinx-contrib/httpdomain/pull/51) has been created to fix the issue,
the introduced hack should be remove if it gets merged and a new
`sphinx-httpdomain` version released.