Migrate Markdown component's React Markdown to Streamdown

[haydenbleasel]

Howdy!

This PR migrates the Markdown component from react-markdown (plus plugins) to Streamdown, a modern markdown renderer purpose-built for AI streaming applications we're building at Vercel. We're using it on a few projects like ChatSDK and thought you might enjoy it too.

Benefits

• Built for AI streaming - Handles incomplete markdown gracefully during real-time content generation
• Fewer dependencies - Replaces 5 packages (react-markdown, rehype-highlight, rehype-raw, rehype-sanitize, remark-gfm) with 1
• Better out-of-box experience - Built-in Tailwind typography, syntax highlighting, and GFM support
• Enhanced security - Native protection against malicious markdown and prompt injection
• CJK language support - Proper emphasis rendering for Chinese, Japanese, and Korean
• Performance optimized - Lots of effort gone in to memoization, LRU caches, etc. to make the fastest and most performant markdown streaming experience.
• Interactive code blocks and tables - Users can download and copy tables / code block data
• Mermaid support - built in support for Mermaid diagrams

I also updated your docs to cover this.

Hope this is a helpful PR, would love to hear your thoughts regardless.

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[ibelick]

hey, thanks for the PR!
I'd prefer to keep the option with react-markdown for now, but we can add a new components that wraps streamdown.
If you want to add it, i'll be happy to review it.