Elide operator file name from documentation

[hirzel]

For example, the operator documentation should not say lale.lib.sklearn.quadratic_discriminant_analysis.QuadraticDiscriminantAnalysis, but rather use the simpler path lale.lib.sklearn.QuadraticDiscriminantAnalysis.

• this makes the "class" line at the top of the page shorter, in some cases preventing it from spilling into the margin
• also, this reduces confusion about where to import from
• furthermore, we would also like to omit the submodules list from the package-level documentation

Hopefully, this can be solved by moving the set_docstring call from the operator.py file to the __init__.py file, and then renaming the file to add a leading underscore, as in, _operator.py.