add help docs and ?

docs/nodes/Edge Detect.md

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+# Edge Detect
+
+Detect edges using Sobel, Prewitt, Laplacian, or Laplacian-of-Gaussian (LoG) operators. Equivalent to gwy_data_field_filter_sobel / gwy_data_field_filter_laplacian.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| edges | DATA_FIELD | Edge-detected output field |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| method | dropdown | sobel | Edge detection operator: sobel, prewitt, laplacian, or log (Laplacian of Gaussian) |
+| sigma | FLOAT | 1.0 | Gaussian smoothing sigma used only for the LoG operator (0.1–10.0) |
+
+## Limitations
+
+- sigma is ignored for sobel, prewitt, and laplacian methods.
+- Sobel and Prewitt return gradient magnitude; Laplacian and LoG return signed second-derivative values.