fix(docs): broken demo video links in identity and payments READMEs

[alelpoan]

Bug Report

Both demo README files contain broken video links that return 404.

demos/identity/README.md
Broken: https://github.com/user-attachments/assets/ccb8b934-5092-4de3-8960-93ee160073aa

demos/payments/README.md
Broken: https://github.com/user-attachments/assets/193b8f66-443f-457f-9370-32835f3b1c85

Both videos are available on YouTube and already used on the official docs site at agentcommercekit.com.

Fix

Addressed in PR #106