add help docs and ?

docs/nodes/Gaussian Filter.md

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+# Gaussian Filter
+
+Apply a Gaussian blur to a DATA_FIELD. Equivalent to gwy_data_field_filter_gaussian.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to filter |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| filtered | DATA_FIELD | Gaussian-blurred field |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| sigma | FLOAT | 1.0 | Standard deviation of the Gaussian kernel in pixels (0.01–50.0) |
+
+## Limitations
+
+- sigma is specified in pixels, not physical units; the effective physical blur depends on pixel size.
+- Large sigma values (> ~20 pixels) are slow due to the large kernel.