Make new `DependentCompositeField` work in entwine forms

[GuySartorelli]

The DependentCompositeField field added in #2056 works in react forms, but doesn't work in entwine forms. This needs to change before we can remove the @internal annotation on it.

Notes

• Since DependentCompositeField is a react component, it is not expected to render the entwine version of fields - i.e. only fields with a react component can be its children, even when used in an entwine form.

Acceptance Criteria

DependentCompositeField:

• correctly renders its child fields in an entwine form
• child field values are correctly populated from the record the form is for
• correctly updated its children when one child's value changes, based on the php dependency callbacks
• submits data correctly
• child fields are validated correctly
• documentation is added to explain how to use this new field
• documentation calls out that values won't automatically be copied from old form fields to new ones after the dependency callback is called - value copying must be done within the callback if it's desirable