CI: build docs in parallel again on CircleCI
CI: build docs in parallel again on CircleCI

The non-parallel build now takes ~22 minutes, so this should fit
comfortably.

ilayn/scipy,zerothi/scipy,anntzer/scipy,tylerjereddy/scipy,andyfaff/scipy,Eric89GXL/scipy,andyfaff/scipy,perimosocordiae/scipy,endolith/scipy,rgommers/scipy,rgommers/scipy,matthew-brett/scipy,anntzer/scipy,e-q/scipy,perimosocordiae/scipy,tylerjereddy/scipy,e-q/scipy,anntzer/scipy,andyfaff/scipy,vigna/scipy,ilayn/scipy,rgommers/scipy,ilayn/scipy,anntzer/scipy,ilayn/scipy,vigna/scipy,Eric89GXL/scipy,mdhaber/scipy,mdhaber/scipy,endolith/scipy,grlee77/scipy,andyfaff/scipy,scipy/scipy,endolith/scipy,zerothi/scipy,grlee77/scipy,andyfaff/scipy,zerothi/scipy,matthew-brett/scipy,anntzer/scipy,perimosocordiae/scipy,anntzer/scipy,mdhaber/scipy,tylerjereddy/scipy,scipy/scipy,endolith/scipy,Stefan-Endres/scipy,zerothi/scipy,Stefan-Endres/scipy,grlee77/scipy,zerothi/scipy,tylerjereddy/scipy,e-q/scipy,scipy/scipy,grlee77/scipy,grlee77/scipy,rgommers/scipy,Stefan-Endres/scipy,Stefan-Endres/scipy,perimosocordiae/scipy,vigna/scipy,WarrenWeckesser/scipy,Eric89GXL/scipy,e-q/scipy,scipy/scipy,rgommers/scipy,matthew-brett/scipy,WarrenWeckesser/scipy,vigna/scipy,ilayn/scipy,andyfaff/scipy,WarrenWeckesser/scipy,mdhaber/scipy,perimosocordiae/scipy,mdhaber/scipy,zerothi/scipy,vigna/scipy,tylerjereddy/scipy,WarrenWeckesser/scipy,e-q/scipy,endolith/scipy,matthew-brett/scipy,Stefan-Endres/scipy,Eric89GXL/scipy,scipy/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,endolith/scipy,mdhaber/scipy,WarrenWeckesser/scipy,matthew-brett/scipy,ilayn/scipy,Eric89GXL/scipy,Eric89GXL/scipy,scipy/scipy,WarrenWeckesser/scipy