add help docs and ?

docs/nodes/Plane Level.md

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+# Plane Level
+
+Fit and subtract a least-squares plane from the data. Supports include/exclude mask fitting for flattening around features, similar to masked plane fitting workflows in Gwyddion.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to level |
+| mask | IMAGE | No | Binary mask for selecting which pixels to include in the plane fit |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| leveled | DATA_FIELD | Field with the fitted plane subtracted |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| masking | dropdown | ignore | How to use the mask: ignore (use all pixels), include (fit plane to masked pixels only), or exclude (exclude masked pixels from fit) |
+
+## Limitations
+
+- When masking is include or exclude, the unmasked/masked region must contain at least three non-collinear pixels to fit a plane.