conf: add a hack to make httpdomain routing table referencable

this aims at eliminating a warning generated by sphinx-build on swh-web's
index, due to the presence of an explicit link to the http-routingtable.html
file (which does not exists at that time, thus generated a warning).

This should be killed once

https://github.com/sphinx-contrib/httpdomain/pull/19

is accepted and released.