Speedup local docs build following ideas from #10187 [skip ci]
Speedup local docs build following ideas from #10187 [skip ci]

- use develop mode to speedup installation
- use parallel mode for Sphinx

larrybradley/astropy,aleksandr-bakanov/astropy,astropy/astropy,pllim/astropy,larrybradley/astropy,saimn/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,astropy/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,mhvk/astropy,lpsinger/astropy,saimn/astropy,astropy/astropy,dhomeier/astropy,astropy/astropy,dhomeier/astropy,saimn/astropy,mhvk/astropy,lpsinger/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,mhvk/astropy,pllim/astropy,StuartLittlefair/astropy,astropy/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,saimn/astropy,dhomeier/astropy,pllim/astropy,StuartLittlefair/astropy,pllim/astropy,lpsinger/astropy,larrybradley/astropy,saimn/astropy,pllim/astropy,larrybradley/astropy,mhvk/astropy