Update README to clarify code block styling and remove stylesheet injection from parser

[samuelstroschein]

Note

Removes automatic CSS injection for syntax highlighting to keep rendered HTML portable and avoid SSR hydration issues.

• Parser: deletes stylesheet injection logic and the related codeBlockDetector; keeps rehype-highlight for code markup without adding <link> tags
• Tests: update to assert no stylesheet injection even when code blocks are present
• Docs: README clarifies styling responsibility, shows how to include a Highlight.js theme or use @opral/markdown-wc/default.css
• Release: adds a changeset with a minor bump for @opral/markdown-wc

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