Update README for sub-packages and intersphinx
Update README for sub-packages and intersphinx

Remove recommendation to use intersphinx (ref #10286).

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