Promote Storybook testing

Storybook testing docs are currently buried in the page contents. This change makes it visible in the right-hand table of contents.

documentation/docs/07-misc/02-testing.md

@@ -246,7 +246,7 @@ test('Component', async () => {
 
 When writing component tests that involve two-way bindings, context or snippet props, it's best to create a wrapper component for your specific test and interact with that. `@testing-library/svelte` contains some [examples](https://testing-library.com/docs/svelte-testing-library/example).
 
-### Component testing with Storybook
+## Component testing with Storybook
 
 [Storybook](https://storybook.js.org) is a tool for developing and documenting UI components, and it can also be used to test your components. They're run with Vitest's browser mode, which renders your components in a real browser for the most realistic testing environment.