DOC document tools StandardizeTransform

[josef-pkt]

just found by chance browsing old PRs. I didn't remember I had written this class.
The mainly internal tools and helper function are not well documented and not easily discoverable.

StandardizeTransform in statsmodels/tools/transform_model.py
(we have transformation functions and classes spread out over different areas for various use cases.)

initially written to fix test failure
general issue #1131

possible usage e.g. in getting better start_params