docs: Document PackMixin and Pack style shorthand on Widget

[arijiiiitttt]

Fixes #4353

What changed

core/src/toga/widgets/base.py

• Added a note to Widget.__init__ docstring explaining that because Widget inherits from PackMixin, Pack style properties can be set directly on any widget instance (e.g. widget.flex = 1 instead of widget.style.flex = 1).

docs/reference/api/widgets/widget.md

• Added a usage example showing the Pack style shorthand syntax.
• Added PackMixin to the reference section so it appears in the generated documentation with its docstring and property list.

Why

Users had no way of knowing that Pack style properties could be set directly on a widget. PackMixin appeared in the base classes of Widget but was not documented or linked anywhere, making it very hard to discover this feature.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file

[freakboy3742]

Thanks for the PR - however, what you've documented here conflicts with the specific advice that I gave in my comment on the original ticket: that ideally, PackMixin wouldn't be documented or surfaced at all. Instead, you've specifically documented PackMixin and included full documentation of the mixin.

We do need to document the relationship between styles, `*kwargs, and style-related properties on widgets - but we need to do that without referencing PackMixin.

[freakboy3742]

The indentation here is required.