Update the link to scikit-learn API page

[Charlie-XIAO]

Change the URL for API in _data/navigation.yml from https://scikit-learn.org/stable/modules/classe to https://scikit-learn.org/stable/api/index. This is because of the theme migration of the scikit-learn main website.

[welcome]

🎉 Welcome to scikit-learn Blog! 🎉
We're really excited to have your input into the project! 💖

If you haven't done so already, please make sure you check out our Contributing Guidelines and Code of Conduct.

[reshamas]

@Charlie-XIAO Thanks for flagging this. The link has been updated.