Handle snippets with multiple code blocks

[kylegach]

What I did

This fix effectively stitches together a snippet that contains multiple code blocks, like this:

```js
// Button.stories.js

import Button from './Button.svelte';
import MarginDecorator from './MarginDecorator.svelte';

export default {
  component: Button,
  decorators: [() => MarginDecorator],
};

// Your stories here.

// Don't forget to use the component you're testing and not the MarginDecorator component

<!-- MarginDecorator.svelte -->

<div>
  <slot />
</div>

<style>
  div {
    margin: 3em;
  }
</style>

Into a unified code block, using the language of the first one:

````mdx
```js
// Button.stories.js

import Button from './Button.svelte';
import MarginDecorator from './MarginDecorator.svelte';

export default {
  component: Button,
  decorators: [() => MarginDecorator],
};

// Your stories here.

// Don't forget to use the component you're testing and not the MarginDecorator component

<!-- MarginDecorator.svelte -->

<div>
  <slot />
</div>

<style>
  div {
    margin: 3em;
  }
</style>

The end result then displays like:

<img width="622" alt="" src="https://github.com/storybookjs/frontpage/assets/486540/e3311c6a-7187-426e-b33d-f9723c1b12c2">

The syntax highlighting for the second portion is not perfect. This solution compromises that for simplicity of implementation, as there are currently only two examples of multi-code block snippets.

### How to test

There are literally only two snippets that meet the criteria requiring this fix and they're both on the `/writing-stories/decorators` page and apply to the Svelte renderer.

1. Open the [deploy preview's Decorators page](https://deploy-preview-644--storybook-frontpage.netlify.app/docs/writing-stories/decorators)
1. Confirm that the snippets look correct
1. Select the Svelte renderer
1. Confirm that these snippets look correct
    1. Under the [**Wrap stories with extra markup** heading](https://deploy-preview-644--storybook-frontpage.netlify.app/docs/writing-stories/decorators#wrap-stories-with-extra-markup)
    1. Under the [**Component decorators** heading](https://deploy-preview-644--storybook-frontpage.netlify.app/docs/writing-stories/decorators#component-decorators)

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[ShaunEvening]

I think that this is a good way to solve the problem as it is but I think we should re-work this page in the most recent docs to split this into two separate snippets with the IfRenderer