add help docs and ?

docs/nodes/Image (Demo).md

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+# Image (Demo)
+
+Load a bundled demo file so you can try the application without providing your own data. The file list is populated from the built-in demo directory at startup.
+
+## Inputs
+
+None.
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| field | DATA_FIELD | Loaded demo field |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| name | dropdown | (first demo file) | Name of the bundled demo file to load |
+| colormap | dropdown | viridis | Colormap applied to the field; hidden when colormap_map is connected |
+
+## Limitations
+
+- Only files present in the bundled demo directory are available; custom files cannot be added here.
+- If no demo files are found, the dropdown shows "(no demo files found)".