[Box] Add support for section element type (#7491)

### WHY are these changes introduced?

Our `Modal` component is being refactored to use our layout primitives
and we need to support `section` as an element type.

### WHAT is this pull request doing?

Adds `section` as accepted element type to the `as` prop for `Box`. 
Updates prop table in style guide with `get-props` script.

### 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)

### 🎩 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
- [x] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

Author: laurkim

.changeset/odd-zebras-yell.md

@@ -0,0 +1,6 @@
+---
+'@shopify/polaris': minor
+'polaris.shopify.com': minor
+---
+
+Added support for `section` element types to `Box`

polaris-react/src/components/Box/Box.tsx

@@ -8,7 +8,7 @@ import {classNames, sanitizeCustomProperties} from '../../utilities/css';
 
 import styles from './Box.scss';
 
-type Element = 'div' | 'span';
+type Element = 'div' | 'span' | 'section';
 
 type Overflow = 'hidden' | 'scroll';