docs: update navigation requests link

[wenxiaojie1]

Summary

• Replace the outdated developers.google.com/web/fundamentals navigation requests link with the current web.dev article.
• Update all four occurrences listed in Broken link in documentation #3536.

Fixes #3536.

Validation

• Confirmed the old URL no longer appears in the four referenced files.
• Confirmed https://web.dev/articles/handling-navigation-requests returns HTTP 200.
• Parsed the two updated JSON schema files with Node.js.
• Ran git diff --check.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.