add help docs and ?

docs/nodes/Cross Section.md

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+# Cross Section
+
+Extract a cross-section height profile along a line between two draggable points on a DATA_FIELD. Equivalent to gwy_data_field_get_profile.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input height field |
+| marker_pair | COORDPAIR | No | Locks both line endpoints from an external coordinate pair |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| profile | LINE | Height profile along the cross-section line |
+| marker_pair | COORDPAIR | Current endpoint coordinates (passthrough for chaining) |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| extend | dropdown | none | none: profile between the two markers; to_edges: extend line to image borders |
+| n_samples | INT | 0 | Number of sample points along the profile (0 = auto, one per pixel diagonal) |
+
+## Limitations
+
+- Profile is sampled using cubic spline interpolation (order 3); sharp step edges may show ringing.
+- Physical x-axis of the output profile is the Euclidean distance in field xy units.