add help docs and ?

docs/nodes/Slope Distribution.md

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+# Slope Distribution
+
+Compute the angular slope distribution of a DATA_FIELD surface. Equivalent to Gwyddion's slope_dist module (slope_dist.c).
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input surface field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| distribution | LINE | Slope distribution histogram |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| distribution | dropdown | theta | Distribution type: theta (inclination angle, probability density in 1/deg), phi (azimuthal direction, weighted by slope², 0–360°), or gradient (slope magnitude, probability density in 1/(z/xy)) |
+| n_bins | INT | 90 | Number of histogram bins (10–1000) |
+
+## Limitations
+
+- The gradient magnitude distribution requires valid physical xy and z units for correct normalization.
+- phi distribution is weighted by slope squared; flat surfaces with near-zero slopes produce a noisy azimuthal distribution.