docs: add spatie/laravel-csp integration example for modern CSP reporting

[audunru]

Summary

• Adds a spatie/laravel-csp code example to the "Modern Reporting API" section showing how to use Directive::REPORT_TO in a Policy class
• Cross-references the existing reporting-endpoints middleware section
• Documents the optional report_uri config for legacy browser fallback (Firefox/Safari), explaining that modern browsers ignore it when report-to is present

Test plan

• Read the updated README and verify the new section renders correctly
• Confirm the spatie snippet style is consistent with the existing legacy example above it

🤖 Generated with Claude Code

[audunru]

🎉 This PR is included in version 2.0.0 🎉

The release is available on GitHub release

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