feat(docs): register FullWidthImage component for lightbox support

FullWidthImage is now available in all MDX pages. Usage:
<FullWidthImage src="..." alt="..." caption="..." />
Clicking any image opens a lightbox overlay. CSS already in custom.css.

Author: divideby0

docs/src/theme/MDXComponents.tsx

@@ -1,8 +1,10 @@
 import React from 'react';
 import MDXComponents from '@theme-original/MDXComponents';
 import Carbon from '@site/src/components/Carbon';
+import FullWidthImage from '@site/src/components/FullWidthImage';
 
 export default {
   ...MDXComponents,
   Carbon,
+  FullWidthImage,
 };