docs: clarify fast refresh README examples

[chenjiahan]

Summary

• clarify that React Fast Refresh requires both runtime injection and source transformation
• update the builtin:swc-loader example to use the broader extension pattern and detectSyntax: 'auto'
• fix example links to point at the current rstack-examples repository

Related Links

• None

Validation

• pre-commit lint:write ran successfully during git commit

[Copilot]

Pull request overview

This PR updates the README to more clearly explain what’s required for React Fast Refresh when using @rspack/plugin-react-refresh, and refreshes the configuration/examples shown to match current guidance and repositories.

Changes:

• Clarifies that Fast Refresh requires both runtime injection (handled by this plugin) and source transformation (handled by a loader).
• Updates the builtin:swc-loader example to use a broader test pattern and detectSyntax: 'auto'.
• Fixes example links to point at the current rstack-examples repository.

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