Reflect new download directory structure (download.osgeo.org/gdal/X.Y.Z) and mentions symlink to CURRENT

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25962 f0d54148-0727-0410-94bb-9a71ac55c965
Mention ARG in format list page (a page on the format itself would be appreciated)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25963 f0d54148-0727-0410-94bb-9a71ac55c965