Finalize lm examples
# What does this PR do?

This PR adds a `run_mlm_wwm` script for an example of MLM with whole word masking, which was the last kind of examples supporter by `run_language_modeling`.

As a result, it moves this script to `contrib/legacy/` and udpates the README to document how to use all the new example scripts.

I also reworked a tiny bit the table of tasks in the main README to include whether or not the examples leverage the Datasets library.