Normalize docstring styles, add type hints and traitlets help

[bollwyvl]

Problem

There are a number of different styles of docstrings for various classes... or none at all. These lead to a lot of warnings in the docs builds, where some of the choices clash with being presented (usefully) with many on a page.

Suggested Improvement

• Normalize on a single docstring style which gives:
  • good documentation when rendered in sphinx
  • good in-client UI when rendered in IPython inspector, JupyterLab, Language Servers, etc.
• Add best-effort type hints
  • these work great in sphinx these days
  • strictly better than "low content" parameters tables that give minimal information beyond typing
• Maybe add some docstring formatting/style/spelling checkers
• Maybe add/update custom inspect.Signature that does a better job of reusing the above
• Maybe write some sphinx transformers to make repetitive headers (e.g. examples, parameters) more useful

References

• follow-on from Overhaul docs toolchain #3673
• see some of the warnings on https://readthedocs.org/projects/ipywidgets/builds/19253722/