Add support for documenting / viewing form widgets

Problem/Motivation

It would be nice to have a list of all form widgets and what entities are using which widget.
This should be available strictly from the data that is already in drupal.
The report should show

1. The name of the widget
2. What field types use it
3. What fields use it
4. what content entities display it

Questions on this:

1. Where should the list live?
2. Should form widgets be documentable?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes