Update documentation on adding a new grid for recent CESM repository arrangement

[billsacks]

As @adamrher pointed out: the documentation in

https://escomp.github.io/cism-docs/cism-in-cesm/versions/master/html/new-grids.html#adding-new-grid-and-mapping-files-in-config-grids-xml

needs to be updated for the new arrangement of repositories in recent versions of CESM.