embed go playground into httpin docs

[ggicci]

Since I've found a way to embed go playground on static websites. One enhancement can be done to the httpin docs is to add sample code with go playground to improve demonstrating how to use httpin package.

[ggicci]

Here's a React component GoPlay.tsx which implements embedding a go playground into docusaurus.

Visit https://ggicci.github.io/httpin/directives/query#run-example as a show case.

[ggicci]

TODO: update documentation to involve more examples.

Or maybe add a tour?