docs: fix learning guide repository link

[stevenchouai]

Summary

• Fixes the Learning Guide's self-link from a 404 GitHub org/repo to the actual public repository.
• Clarifies that the linked repository is the unofficial research repo readers are currently viewing.

Validation

• npm run docs:build
• curl check: old URL returns 404; new URL returns 200

Notes

• Docs-only change; privacy scan found only expected generic token/URL wording, no secrets.