docs: add empty storybook .preview.js file with comments explaining h…

…ow/when it should be used

.storybook/preview.js

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+/**
+ * Read https://storybook.js.org/docs/react/configure/overview#configure-story-rendering
+ * for more information about the purpose of this file.
+ *
+ * Use preview.js for global code (such as CSS imports or JavaScript mocks)
+ * that applies to all stories. For example, `import thirdPartyCss.css`.
+ *
+ * This file can have three exports:
+ * - decorators - an array of global decorators
+ * - parameters - an object of global parameters
+ * - globalTypes - definition of globalTypes
+ */
+
+/**
+ * Decorators
+ *
+ * A decorator is a way to wrap a story in extra “rendering” functionality.
+ *
+ * Example:
+ *
+ * import React from 'react';
+ * export const decorators = [(Story) => <div style={{ margin: '3em' }}><Story/></div>];
+ *
+ * Each story throughout the library will be wrapped in a div with a margin of 3
+ */
+
+/**
+ * Parameters
+ *
+ * Most Storybook addons are configured via a parameter-based API.
+ * You can set global parameters in this file
+ *
+ * export const parameters = {
+ *   backgrounds: {
+ *     values: [
+ *       { name: 'red', value: '#f00' },
+ *       { name: 'green', value: '#0f0' },
+ *     ],
+ *   },
+ * };
+ *
+ * With backgrounds, you configure the list of backgrounds that every story can render in.
+ */
+
+/**
+ * Global Types
+ *
+ * Global Types allow you to add your own toolbars by creating
+ * globalTypes with a toolbar annotation:
+ *
+ * For example:
+ *
+ * export const globalTypes = {
+ *   theme: {
+ *     name: 'Theme',
+ *     description: 'Global theme for components',
+ *     defaultValue: 'light',
+ *     toolbar: {
+ *       icon: 'circlehollow',
+ *       // array of plain string values or MenuItem shape
+ *       items: ['light', 'dark'],
+ *       },
+ *     },
+ *   };
+ *
+ * Will add a new dropdown in your toolbar with options light and dark.
+ **/