Add explanation of Router Playground

[imurata]

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

• I agree to follow this project's Code of Conduct
• I agree to abide by the Community Expectations

[sumimakito]

Thanks for reporting. We will internally track this improvement by ticket KM-651.