docs(aggregators): add API key setup script and update documentation
- Add 5-create-api-key.sh for generating aggregator API keys
- Update aggregator-setup README with step 5 documentation
- Clarify that domain verification (.well-known) is optional
- Document both PDS handle and custom domain options
- Update kagi-news README for API key authentication flow
- Replace old password-based auth references with API key auth
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>