Improve docs for `EntityEmbedder` wrapper

[ablaom]

There is no reference to this wrapper at all in the docs.

Also, the doc string is confusing. What is the model that is bound to the machine? (It is the output of model = EntityEmbedder(model=atomic_model).)

[ablaom]

By the way, my undestanding is this wrapper turns the atomic supervised model into an unsupervised one (with training target), so that it can be used in a pipeline and be interpreted as a transformer and not a supervised model. Perhaps the example in the docs string could explicitly give a pipeline example.