Add FAQ entry about process.env.STORYBOOK

This adds a FAQ entry titled "How can my code detect if it is running in Storybook?". The solution of checking `process.env.STORYBOOK` was recommended by @shilman.

Closes #16343.

docs/workflows/faq.md

@@ -342,3 +342,32 @@ Out of the box, Storybook provides syntax highlighting for a set of languages (e
 </div>
 
 Applying this small change will enable you to add syntax highlight for SCSS or any other language available.
+
+### How can my code detect if it is running in Storybook?
+
+You can do this by checking the value of `process.env.STORYBOOK`, which will
+equal `'true'` when running in Storybook. Be careful — `process` may be
+undefined when your code runs outside of Storybook.
+
+```javascript
+export function isRunningInStorybook() {
+  try {
+    if (process.env.STORYBOOK) return true;
+  } catch {
+    // A ReferenceError will be thrown if process is undefined
+  }
+
+  return false;
+}
+```
+
+This works because Babel replaces `process.env.STORYBOOK` with the value of the
+`STORYBOOK` environment variable. Because this is done through a Babel plugin,
+the following will **NOT** work:
+
+```javascript
+export function isRunningInStorybook(): boolean {
+  return typeof process?.env?.STORYBOOK !== 'undefined';
+  // ReferenceError: process is not defined
+}
+```