UX: Update banner image field to use help text (#30666)

### What's changed?
- Add support for`@helpText` in the FormKit component.
- Updated the about config page to replace `@description` with
`@helpText` for the banner image field

### Before
<img width="525" alt="image"
src="https://github.com/user-attachments/assets/e8cdcca7-1b46-406f-8b6b-815e65dd5d9c"
/>

### After
<img width="256" alt="image"
src="https://github.com/user-attachments/assets/8656b866-5654-4f67-8515-f58a7b2b14c0"
/>


Internal: /t/138358/2

config/locales/client.en.yml

@@ -5781,7 +5781,7 @@ en:
           community_title_help: "Short description, shown in the browser tab, for key pages such as categories and topic lists."
           banner_image: "Banner image"
           banner_image_help: |
-            This will be used on your About page. Recommended size: 1100x300px. Accepted types: JPG, PNG, and SVG up to 10MB.
+            Recommended: 1100x300px. Formats: JPG, PNG, SVG (max 10MB).
           contact_information: "Contact information"
           community_owner: "Community owner"
           community_owner_placeholder: "Johnny Smith"