Document widget_toolkit theme fields

[DDavidPrime]

As a developer whenever I want to customize a widget theme from any of the widget_toolkit packages, I have to go through a list of the undocumented fields and try the most meaningful ones one by one. Currently, all those fields lack documentation that indicates which part of a specific widget is being affected. The goal of this ticket is to update all widget_toolkit packages to include documentation on those fields.

This ticket serves as an epic.

ACs:
The following packages should be covered:

• widget_toolkit
• widget_toolkit_qr
• widget_toolkit_pin
• widget_toolkit_otp
• widget_toolkit_biometrics

Estimate: 40h