docs: explain shadow DOM style injection

[razchiriac]

Summary

• Documents that styles from the host page do not automatically apply inside shadow: \"open\" or shadow: \"closed\" components.
• Adds an example of extending the generated custom element and appending a stylesheet link to the shadow root in connectedCallback.
• Notes the Vite/Tailwind pattern of emitting a CSS file and linking to the built asset.

Closes #172.

Test plan

• pnpm dlx prettier --check docs/api.md

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