Disable parallel build of Sphinx documentation
Disable parallel build of Sphinx documentation

We are using some Sphinx extensions that are not safe w.r.t. parallel
reaading.

coq/coq,Zimmi48/coq,ejgallego/coq,coq/coq,SkySkimmer/coq,gares/coq,Zimmi48/coq,JasonGross/coq,silene/coq,SkySkimmer/coq,coq/coq,ejgallego/coq,ejgallego/coq,silene/coq,coq/coq,JasonGross/coq,silene/coq,ejgallego/coq,SkySkimmer/coq,JasonGross/coq,gares/coq,Zimmi48/coq,gares/coq,SkySkimmer/coq,gares/coq,JasonGross/coq,Zimmi48/coq,gares/coq