Add redirection from https://lhcb-gauss.docs.cern.ch/ to https://lhcb-gauss.docs.cern.ch/master/

By accident I noticed that the URL https://lhcb-gauss.docs.cern.ch/ does not work.

While https://lhcb-gauss.docs.cern.ch/index.html redirects to https://lhcb-gauss.docs.cern.ch/master/index.html, https://lhcb-gauss.docs.cern.ch/ just gives a 404 error.

This, if I guessed correctly, should fix the redirect.