add help docs and ?

docs/nodes/Fix Zero.md

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+# Fix Zero
+
+Shift data so that the minimum, mean, or median value becomes zero. Equivalent to fix_zero in Gwyddion's level.c.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| zeroed | DATA_FIELD | Field shifted so the chosen reference is at zero |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| method | dropdown | min | Reference value to set to zero: min (lowest pixel), mean (average), or median |
+
+## Limitations
+
+- None.