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

[alelpoan]

Fix broken demo video links in identity and payments READMEs.

Both README files referenced github.com/user-attachments/assets/...video links that return 404. Replaced with clickable YouTube preview thumbnails linking to the correct videos already used on the official docs site at agentcommercekit.com.

Note: GitHub Markdown does not support <iframe> embeds, so a clickable thumbnail image is the standard approach for linking to videos in READMEs.

Changes:
demos/identity/README.md: broken user-attachments asset link →
YouTube thumbnail preview for https://www.youtube.com/watch?v=CDJ8pm_KlOw
demos/payments/README.md: broken user-attachments asset link →
YouTube thumbnail preview for https://www.youtube.com/watch?v=49VaG1-u1i0

Test plan:
Documentation-only change; no code paths affected.

Summary by CodeRabbit

• Documentation
  • Embedded YouTube demo videos directly in the identity and payments demo guides, replacing previous media references for improved accessibility and streamlined user experience when reviewing demo content.

Closes #107

Screenshots

Before (broken 404 video):

After (clickable YouTube preview):

Screenshots

Before (broken 404 video):

After (clickable YouTube preview):

[coderabbitai]

Walkthrough

This PR updates two demo README files by replacing previous video references with embedded, centered YouTube player links. Both the identity and payments demo READMEs now display clickable YouTube thumbnail embeds in their respective "Demo Video" sections using HTML <p align="center"> blocks.

Changes

Demo Video Documentation Updates

Layer / File(s)	Summary
YouTube embedded video documentation demos/identity/README.md, demos/payments/README.md	Identity and payments demo READMEs update "Demo Video" sections by replacing previous media/attachment references with centered HTML <p align="center"> blocks containing YouTube video links and thumbnails.

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: replacing broken demo video links in two README files (identity and payments) with working YouTube thumbnails.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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