Update documentation theme

[yalinli2]

This PR will change QSDsan's documentation theme from the previous default Read the Docs to the more modern Furo

I originally liked Press best because of the top navigation bar, but it just won't work with the tutorials in Jupyter Notebook, look like due to this issue, and it has not been updated for more than half a year.

A couple of reasons that Furo is chosen:

• Large user base and under active development
• Full navigation bar on the left
• First-order header TOC on the right
• Eye-catching search bar
• Have dark themes
• Looks nice on phone (i.e., narrower windows)