Add explanation of Router Playground

### Where is the problem?

https://docs.konghq.com/gateway/latest/reference/expressions-language/language-references/

### What is your suggestion?

Now if I try to check the detailed usage in Router Playground, I will be redirected to the following page.
https://docs.konghq.com/gateway/latest/reference/expressions-language/language-references/

The page I jump to does not explain Router Playground, so it is difficult to understand how to use it.
I would be happy to have a description of Router Playground.

### Code of Conduct and Community Expectations

- [X] I agree to follow this project's Code of Conduct
- [X] I agree to abide by the Community Expectations