feat(website): add agpl footer notice

[sambacha]

📝 Summary

add AGPL footer notice

⛱ Motivation and Context

AGPL Compliance for making available source

📚 References

Meeting the AGPL requirements for manifold finance usage of relay:

The software included in this service contains copyrighted software that is licensed under the AGPL. A copy of that license is included in this document available at <https://github.com/manifoldfinance/mev-boost-relay/LICENSE>. You may obtain the complete Corresponding Source code from us for a period of three years after our last shipment of this product, which will be no earlier than 2025-09-15

You may also find a copy of the source at <https://github.com/manifoldfinance/mev-boost-relay>

This offer is valid to anyone in receipt of this information.

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✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md