Add user-facing doc about deprecation/removal
Add user-facing doc about deprecation/removal

lpsinger/astropy,mhvk/astropy,mhvk/astropy,StuartLittlefair/astropy,lpsinger/astropy,mhvk/astropy,pllim/astropy,larrybradley/astropy,StuartLittlefair/astropy,astropy/astropy,astropy/astropy,astropy/astropy,saimn/astropy,pllim/astropy,larrybradley/astropy,saimn/astropy,larrybradley/astropy,lpsinger/astropy,astropy/astropy,saimn/astropy,astropy/astropy,StuartLittlefair/astropy,mhvk/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,saimn/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,saimn/astropy,lpsinger/astropy,pllim/astropy,larrybradley/astropy,pllim/astropy,pllim/astropy