Deprecate/consolidate docs outside sdr-documentation repo

[mnyrop]

E.g., https://github.com/NYULibraries/spatial_data_repository/wiki
https://docs.google.com/spreadsheets/d/1cEoenvQXXhaA6mCGs4gtJlEN_AasXOArnFJa-561Kq0/edit#gid=0