move gdalcompare.py to scripts, and add docs

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26383 f0d54148-0727-0410-94bb-9a71ac55c965
Add link to gdalcompare

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26384 f0d54148-0727-0410-94bb-9a71ac55c965