Add an example of `EarthLocation.get_site_names()`
Add an example of `EarthLocation.get_site_names()`

There is now a tested example of calling
`astropy.coordinates.earth.EarthLocation.get_site_names()` in
`docs/coordinates/index.rst`.

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