Overview level selection: accept a 'only' suffix after the overview level in OVERVIEW_LEVEL open option (#5688)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27862 f0d54148-0727-0410-94bb-9a71ac55c965
add link to nas driver page [ci skip]

git-svn-id: https://svn.osgeo.org/gdal/trunk@27863 f0d54148-0727-0410-94bb-9a71ac55c965