Add an interactive-visualization (widgets) tutorial

[TomaSusi]

Gap: abtem/visualize/widgets.py (ImageGUI, ScatterGUI, LinesGUI, BaseGUI) is only touched indirectly by the interactions gallery example; there is no explanation of the widget system, its options, or usage patterns.

Why it matters: Interactive exploration of ensemble/4D data is one of abTEM's distinguishing conveniences, but users can't discover it from the docs.

Existing draft: docs/user_guide/tutorials/visualizations.ipynb is complete (44 cells, executed) but commented out of _toc.yml — likely the best starting point: re-verify it against current abTEM, extend the widget section, and re-include it.

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From a docs coverage audit (July 2026) comparing the abTEM public API against documentation content.

🤖 Generated with Claude Code

[TomaSusi]

Left this one alone for now. The natural path is extending the existing (complete, executed) visualizations.ipynb with a section on abtem.visualize.widgets (ImageGUI, ScatterGUI, LinesGUI) and un-hiding it in _toc.yml — but that notebook is one of four (alongside epie, custom_simulation_objects, plasmons) that a prior session deliberately kept hidden per an explicit decision from the maintainer, with several other issues referencing them as draft material. Re-including or editing it needs a direct call on whether that decision still stands, rather than a docs-audit session overriding it unilaterally.