Documentation does not build if https://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd is unavailable
(currently the case)
@gyuris or @ede123 do you know of a way to not depend upon that website when building our documentation? The dtd is available from other sources, too. I don't know how to indicate which one to use, though.
We could include it in the repo or install it from some package, probably.