Add Widgetbook setup with theme configuration, analysis options, and initial component use cases

widgetbook/lib/button_m3e/INDEX-button_m3e.md

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+# Widgetbook Index — button_m3e
+
+This index lists the ButtonM3E component and all provided Widgetbook variants.
+
+## Components and Variants
+
+- ButtonM3E
+  - default
+  - filled
+  - tonal
+  - elevated
+  - outlined
+  - text
+  - disabled
+  - focused
+  - selected
+  - with_icon
+  - long_text
+  - empty_label
+
+## Notes
+- Critical parameters are exposed via knobs: size, shape, enabled, toggleable, selected, label text, icon selection.
+- Interaction states can be previewed using knobs and, where needed, simulated via WidgetStatesController (focused, selected).
+- Themes are injected globally by the Widgetbook app.