sandbox md nit (Shopify#7428)

- update to use md convention to list sandbox route in navigation
- exclude /sandbox from catch all template
- changeset
- sandbox/index.md --> sandbox.md
- remove unnecessary description

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### WHY are these changes introduced?

Fixes #0000 <!-- link to issue if one exists -->

<!--
  Context about the problem that’s being addressed.
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### WHAT is this pull request doing?

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  Summary of the changes committed.

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      <summary>Summary of your gif(s)</summary>
      <img src="..." alt="Description of what the gif shows">
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<!-- ℹ️ Delete the following for small / trivial changes -->

### How to 🎩

🖥 [Local development
instructions](https://github.com/Shopify/polaris/blob/main/README.md#local-development)
🗒 [General tophatting
guidelines](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md)
📄 [Changelog
guidelines](https://github.com/Shopify/polaris/blob/main/.github/CONTRIBUTING.md#changelog)

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  Give as much information as needed to experiment with the component
  in the playground.
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<details>
<summary>Copy-paste this code in
<code>playground/Playground.tsx</code>:</summary>

```jsx
import React from 'react';
import {Page} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      {/* Add the code you want to test in here */}
    </Page>
  );
}
```

</details>

### 🎩 checklist

- [ ] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [ ] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [ ] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [ ] Updated the component's `README.md` with documentation changes
- [ ] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

polaris.shopify.com/content/sandbox.md

@@ -1,7 +1,6 @@
 ---
 title: Sandbox
 order: 10
-description: Cards are used to group similar concepts and tasks together to make Shopify easier for merchants to scan, read, and get things done.
 url: /sandbox?code=N4Igxg9gJgpiBcIA8BhAhgJygHQHYAJ8BnGSXKTATwDEIIAXGDAQTHoEsJciBeYAbWBlGuevHwByACLsiAW1lEJAXwC6yvIQAOGdnKq0GTVhy58hXEWMkBRAB5aIGevgBKMR85UbcAPk34qJhQAHQAEjBosBj4aGyc3HyC%2BMIwouISACoMaAA2%2BADKeTBK%2BGrK%2BBz0uTA82CBFNUT1-gSEgQAKEI4AbkwB7bHxfXwAZnkkPoOEcRw9aPROfAPTSABCAK70iwRQsmC5EEQbGDD4Wrlo7H4r0-gAauwwAO6Fxc1td0gA9JvbXK07lNplwUIcSHwABQASnwPF8%2BGAymBhEBqxMCQAMrIXOxGHJeAILKI0tYJABxDBHIhvJoqAA0iNS6UkADkYC5GiUVOp8N80YQfl1ekw0T90FhwpFomKJaECqRTDdPoFMjA7PQUJYrrhRbd8ABNCAbFJoAgbEjEd74U6eeg0xbEGBna6jJz6JWxABGxpclGNMTAFsWciYREAmAT6pzRGletAkKD4LgpXSMXRoYgbMAAC1iNKI1ogfRiHFDjK9lBS2bNutyjKc%2BAr%2BqI9DQo1GIRWPzVGq1oh1era4uCIQV8QBASCkrHnqqNTqIGyrfyXJpa1OaAA1lAIM9cC0u%2B4iAGwDBsS39adjydT6y0KHzERrgBzDaXDAZAs1CSMi4nPKfu83j6niMAEkk%2BqEMAkHtF%2BJQZAA8lgYY-jBMxyMaLISAAqgUUgACQAAwhIRhGoSqdwnLkGS2k49rfNGKH0jByjMRR7TQexgxwUQGTuPQJzcORdyDPemFkgAtLhBEAEwAKzEaRwkiYQVE0R4dFEN8pwCRgQlsSprH6uol5pNEACS%2BJQqBciwvCiIwWQLaIjxjJiRsoiMlRFQ8PgNkANwwTpgn4JCaGBEeJ5njiISWWB4WqRguR8N5CVDKeRBPl67C5HilCYmgXowMlwAAAaPC8tIlPgboxPhwA8copUoip%2BACq1SAFK2YCbu1rWBF1cSbrF%2BI1TluS%2BA17zKD8g09SNYF9R1c3DXFciTe5ogzd8K0LetaWzd1vXhT8kU3tFLZ7UthDQoF7HIvq-KTt8cqjoqCSyiOM4JJUeLzvUUiRMMCwwImtHOM0ICZl6UAbKD7VIOe9BLYjMVrb4HRoP6WxED8SNXfqqOXejS55FVa6Vig1a4LWeNo-iCPfEjYovV970TkOr3fcmc61PUrJGAeKpID2mratcg53Jk2ZnDxNoaRDsSnLE8w5YVNRJrguSVuwoz4NjMQtk4Lo0jzMuFNm3S65Q%2BoXGaSYxNm7DPjLGCdsL3yi32rYSxgLNc%2BzyrDlgvggPSID0DLoZEAg-AgIcYDFPAaQgKo4fPOwUCRzH8D8OoQA
 status:
   value: Alpha