[DOC] - Remove extension system warning

[kcpevey]

Preliminary Checks

• This issue is not a question, feature request, RFC, or anything other than a bug report. Please post those things in GitHub Discussions: https://github.com/nebari-dev/nebari/discussions

Summary

We've had the extension system since 2023.9.1. Lots of people use it. Its time to remove the warning banner on https://www.nebari.dev/docs/how-tos/nebari-extension-system.