PUBDEV-5756: Add Python example to cross-validation topic (#2725)
PUBDEV-5756: Add Python example to cross-validation topic (#2725)

- Added Python examples to the cross-validation topic. Make those
similar to the R examples.
h2oai/h2o-3,h2oai/h2o-dev,michalkurka/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-dev,michalkurka/h2o-3,h2oai/h2o-dev,h2oai/h2o-dev,h2oai/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,h2oai/h2o-dev,h2oai/h2o-3