add help docs and ?

docs/nodes/Statistics.md

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+# Statistics
+
+Compute basic surface statistics: min, max, mean, RMS roughness, median, and skewness. Equivalent to gwy_data_field_get_min/max/avg/rms.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to analyze |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| stats | RECORD_TABLE | Table of statistical quantities with values and units |
+
+## Controls
+
+None.
+
+## Limitations
+
+- None.