GHA - the htmlproofer reports failures on github.com urls

[pmauduit]

htmlproofer being used in the GHA (https://github.com/georchestra/georchestra.github.io/blob/master/.github/workflows/build_and_test_website.yml#L21-L27) reports errors on some github-related urls and fails:

[...]
* At /github/workspace/_site/fr/documentation.html:147:

  External link https://georchestra-user-guide.readthedocs.io/en/latest/setup/datafeeder/ failed (status code 404)

* At /github/workspace/_site/fr/documentation.html:187:

  External link https://github.com/georchestra/data-api failed (status code 403)

* At /github/workspace/_site/fr/documentation.html:200:

  External link https://github.com/NielsCharlier/geofence failed (status code 403)

* At /github/workspace/_site/fr/documentation.html:209:

  External link https://github.com/georchestra/georchestra/tree/master/console failed (status code 403)