Detecting multi-touch gestures on mobile

### What is the problem or limitation you are having?

Sorry if I've missed existing documentation.

I'm trying to understand how multi-touch gestures on mobile are handled, especially when widgets don't have something built-in to handle them. For example, what if I want to zoom in on an image/canvas by using a two-finger-pinch type gesture? What if I want to rotate an image using a two-finger-twist gesture? Are the individual finger positions made available to us through toga?

### Describe the solution you'd like

Documentation on how multi-touch user inputs are accessed on mobile

### Describe alternatives you've considered

Maybe an external library to track gestures?

### Additional context

_No response_

No files or tests are named. Start by reviewing Toga's existing mobile input and widget documentation, then verify whether multi-touch finger positions or gesture events are exposed; done means documenting the supported mobile behavior and any limitations or alternatives.

Written by the indexing model from the issue text.