docs: fix broken contributor link - #1022

Summary

• Updates the Jonathan Dan contributor reference in docs/whats_new.rst from a 404 GitHub profile URL to the active jon-dan profile.
• Keeps the existing release note attribution and changes only the broken target URL.
• Addresses the single 404 reported in Broken links detected in documentation #1017.

Validation

• git diff --check passed.
• Invoke-WebRequest -Method Head https://github.com/jon-dan returned HTTP 200.
• Invoke-WebRequest -Method Head https://github.com/danjjl returned HTTP 404, matching the reported broken link.
• lychee --version was not available locally, so the full project link-check workflow was not run on this machine.

Notes

• Related issue: Broken links detected in documentation #1017.
• No runtime code or compatibility impact.
• No follow-up work expected for this one-link fix.