add a few more nodes

docs/nodes/Zero Maximum.md

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+# Zero Maximum
+
+Shift all values so the maximum is exactly zero. All resulting values will be zero or negative.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to level |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| leveled | DATA_FIELD | Field with maximum subtracted |
+
+## Notes
+
+- Equivalent to subtracting the global maximum from every pixel.
+- Useful when the highest point should represent the zero reference.