docs(widgets): add widget system section index

widgets/README.md

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+# Widgets
+
+Widgets are the building blocks of the canvas and the Control Panel. Each widget is a small,
+self-contained app: its own data, its own presentation settings, and optionally a live data binding.
+
+## Two families
+
+| Family | Source | Rendered as | Examples |
+| --- | --- | --- | --- |
+| **Vue widgets** | `w4c-quasar/src/components/widgets/` | native Vue components | List, Tree, Table, Board |
+| **React widgets** | `w4c-quasar/src/components/excalidraw/` | Excalidraw `embeddable` elements | Chart, Visualization, Markdown, Diagram, Form, Code |
+
+## Where widgets render
+
+- **Excalidraw canvas** — a widget is an `embeddable` element. Its frame, fill and stroke are drawn by
+  the node itself, so a widget never draws its own border.
+- **Control Panel** (`/control-panel`) — a grid of widget cells. Each cell renders the same widget
+  type, with the style stored per cell.
+
+## The registry
+
+`src/components/control-panel/widgetRegistry.ts` is the catalog: every type with its label, icon,
+category and size constraints. Widgets **self-seed** their metadata when empty, so adding a new one
+always shows a sensible default instead of an empty frame.
+
+## Data: static or bound
+
+- **Static** — the data lives in the widget's own metadata (columns and rows for a table, cards for a
+  board). Everything is editable in place.
+- **Bound** — the widget carries a data binding instead. Bound widgets are read-only and refresh from
+  their source.
+
+## Pages
+
+- [Table](#/docs/widgets/table)