Adding missing md files for node preview descriptions (#7806)

* Adding missing md files

* Adding md file references to a few I missed.

Author: bencloward

com.unity.sg2/Documentation~/previews/BlendWaterNormal.md

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+## Description
+Blends the water's tangent space pixel normal with the world space vertex normals
+
+## Inputs
+**NormalTS** - the water's pixel normals in tangent space
+**NormalWS** - the water's vertex normals in world space
+
+## Outputs
+**NormalWS** - the combined vertex and pixel normal in world space

com.unity.sg2/Documentation~/previews/Boolean.md

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+## Description
+A true/false check box
+
+## Output
+**Out** - A constant true or false value.

com.unity.sg2/Documentation~/previews/BuiltinCorneaIOR.md

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+## Description
+Brings in the Cornea IOR data.
+
+## Output
+**Builtin Cornea IOR** - the cornea IOR data

com.unity.sg2/Documentation~/previews/BuiltinIrisPlaneOffset.md

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+## Description
+Brings in the Iris Plane Offset data for the eye.
+
+## Output
+**BuiltinIrisPlaneOffset** - the Iris Plane Offset data for the eye

com.unity.sg2/Documentation~/previews/BuiltinIrisRadius.md

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+## Description
+Brings in the Eye's Iris Radius value.
+
+## Output
+**BuiltinIrisPlaneOffset** - the eye's iris radius

com.unity.sg2/Documentation~/previews/ChannelMixer.md

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+## Description
+Adjusts the input RGB value with weighted values.
+
+## Inputs
+**In** - the color or vector to adjust
+
+## Output
+**Out** - the adjusted color
+
+## Controls
+**RGB Tabs** - selects the output channel to adjust
+**RGB Sliders** - Controls contribution of input red, green, or blue channels to the output channel

com.unity.sg2/Documentation~/previews/ComputeDeformation.md

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+## Description
+Passes computed deformation data to the vertex shader
+
+## Output
+**Deformed Position** - Position data with deformation added
+**Deformed Normal** - Normal data with deformation added
+**Deformed Tangent** - Tangent data with deformation added

com.unity.sg2/Documentation~/previews/EvaluateWaterSimulationDisplacement.md renamed to com.unity.sg2/Documentation~/previews/EvaluateWaterDisplacement.md

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+## Description
+A single channel value with a decimal
+
+## Output
+**Out** - A single channel value with a decimal

com.unity.sg2/Documentation~/previews/Float.md

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+## Description
+A single channel value without a decimal
+
+## Output
+**Out** - A single channel value without a decimal