add help docs and ?

docs/nodes/Median Filter.md

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+# Median Filter
+
+Apply a median filter to a DATA_FIELD. Equivalent to gwy_data_field_filter_median.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to filter |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| filtered | DATA_FIELD | Median-filtered field |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| size | INT | 3 | Kernel size (side length) in pixels; odd values only (1–21) |
+
+## Limitations
+
+- The median filter is applied with a square kernel; non-square (e.g. rectangular) kernels are not supported.
+- Large kernel sizes are significantly slower than the Gaussian filter for the same smoothing extent.