add help docs and ?

docs/nodes/Preview.md

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+# Preview
+
+Display an IMAGE or DATA_FIELD as a coloured thumbnail in the node panel.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| input | ANNOTATION_SOURCE (DATA_FIELD or IMAGE) | No | Field or image to display |
+| colormap_map | COLORMAP | No | Colormap socket; overrides the colormap widget |
+
+## Outputs
+
+None.
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| colormap | dropdown | auto | Colormap used when rendering a DATA_FIELD or grayscale IMAGE; hidden when colormap_map is connected |
+
+## Limitations
+
+- When no input is connected, an error is raised at execution time.
+- Connecting a FILE_PATH socket (instead of DATA_FIELD) raises a type error; ensure the correct socket is connected.