Components.AI: Document and sample markdown-to-rich-node integration

### Is your feature request related to a problem? Please describe the problem.

Markdown intentionally remains outside the core library, but applications need a supported end-to-end integration pattern.

### Describe the solution you'd like

Add documentation and a sample showing parser selection, conversion into `RichTextNode`, safe policy configuration, streaming updates, and rendering through the built-in extensible node renderer.

### Additional context

Leaf task for #68227 requirement J3 and decision D1.

Start by reading requirement J3 and decision D1 in #68227, then locate the RichTextNode and built-in extensible node renderer entry points. The documentation and sample should cover parser selection, conversion, safe policy configuration, streaming updates, and rendering.

Written by the indexing model from the issue text.