Document MarkdownViewer component

[hyyan]

New component in 25.11 for rendering markdown with streaming support.

Key features to document:

• Progressive rendering (setProgressiveRender, setRenderSpeed)
• Auto-scroll behavior (setAutoScroll)
• Streaming methods (append, stop, flush, clear)
• whenRenderComplete() for knowing when rendering finishes
• Prism.js integration for syntax highlighting

webforj/webforj#1165

[hyyan]

https://github.com/webforj/built-with-webforj/tree/main/webforj-ghostai