More examples about rate-based whole brain modeling

[chaoming0625]

Please:

• [x] Check for duplicate requests.
• [x] Describe your goal, and if possible provide a code snippet with a motivating example.

Rate-based whole brain modeling is attractive for brain modeing users. However, the support (including documentaion and examples) of it in BrainPy is still limited. Maybe we should provide more support on this topic.