added docs for variables

docs/_guide/variables.md

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+---
+layout: page
+title: Variables
+nav_order: 7.1
+---
+
+# Variables
+
+Variables are a way for operators to provide additional information to other operators in the scene.
+
+For example, the [`modulo1D`](/raytk/reference/operators/filter/modulo1D) operator repeats slices of space along an axis. It offers a variable indicating which slice is being rendered, so that its input can vary settings per-slice.
+
+Another example is the [`assignColor`](/raytk/reference/operators/filter/assignColor) operator, which adds a color setting to an SDF result. It offers a variable to access the SDF result produced by its first input, so that the "Color Field" input can base the color on something like the SDF's UV coordinates.
+
+Variables can be created in one of two ways:
+
+1. Selecting the operator providing the variable, and using the editor tools menu (`alt+shift+r`), with the "Reference Variable" sub-menu.
+2. Clicking one of the buttons on the providing operator's "Variables" parameter page.
+
+Doing this will create a [`variableReference`](/raytk/reference/operators/utility/variableReference) operator, which access the value of the selected variable from the selected operator. This operator produces whatever type of value that variable uses, either in its entirety or just a single field within that value. For example, a vector variable can be accessed to produce either the whole vector, or just the Y part of the vector, depending on the settings on the `variableReference`.
+
+## Tutorial Video
+
+<iframe width="560" height="315" src="https://www.youtube.com/embed/eqqOlSEk0YA" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>