Add index for coqdoc.
Add index for coqdoc.

Fixes #12545.

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