Improvements in Quick-start for Ranking

[gabrielspmoreira]

This PR adds some improvements to the Quick-start for ranking scripts and documentation

In preprocessing.py:

• Adds support to target encoding features, configurable through these new CLI arguments: --target_encoding_features, --target_encoding_targets, --target_encoding_kfold, --target_encoding_smoothing.

In ranking.py:

• Adds support to selecting some columns to keep (--keep_columns) or remove (--ignore_columns) from at dataloading / training / evaluation.

This PR also converts those scripts to Python modules, to make it easier to import/extend their classes and to test them. So now, instead of being run like python preprocessing.py --args ..., they need to be run as a Python module, e.g.

cd /Merlin/examples/
python -m quick_start.scripts.preproc.preprocessing --args ...

[github-actions[bot]]

Documentation preview

https://nvidia-merlin.github.io/Merlin/review/pr-1014

[rnyak]

@gabrielspmoreira I approved in case you want to merge once you push your final changes.