Mention ARG driver in Various Supported Raster Formats.

git-svn-id: https://svn.osgeo.org/gdal/trunk@28224 f0d54148-0727-0410-94bb-9a71ac55c965
Add link to documentation of ARG -- Azavea Raster Grid driver

git-svn-id: https://svn.osgeo.org/gdal/trunk@28225 f0d54148-0727-0410-94bb-9a71ac55c965