[Docs]: 缺少「坐标系- radar」文档

[interstellarmt]

📍 Which page are you reading?

https://g2.antv.antgroup.com/examples/general/radar/#parallel-radar

💭 What's on your mind?

Other suggestions / 其他建议

🤔 Tell us more

坐标系 - radar 的文档缺失

💡 Got any suggestions?

No response

[mingcheng]

We can create a new document and add the missing ones.

@mingcheng Give it to me