[Layout foundations] Remove ContentBlock (#7746)

### WHY are these changes introduced?

Resolves #7737.
Feedback from our usability testing sessions showed that the purpose of
ContentBlock is unclear and does not provide much value. We've decided
to remove this component, as there are ways to adjust width on children
with our other layout primitives.

### WHAT is this pull request doing?

Removes ContentBlock and the page/examples in the style guide.

<!-- 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)

<!--
  Give as much information as needed to experiment with the component
  in the playground.
-->

<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

.changeset/kind-boats-trade.md

@@ -0,0 +1,6 @@
+---
+'@shopify/polaris': minor
+'polaris.shopify.com': minor
+---
+
+Removed `ContentBlock` and its examples in the style guide

polaris-react/src/index.ts

@@ -128,9 +128,6 @@ export type {ComboboxProps} from './components/Combobox';
 export {Connected} from './components/Connected';
 export type {ConnectedProps} from './components/Connected';
 
-export {ContentBlock} from './components/ContentBlock';
-export type {ContentBlockProps} from './components/ContentBlock';
-
 export {ContextualSaveBar} from './components/ContextualSaveBar';
 export type {ContextualSaveBarProps} from './components/ContextualSaveBar';