Documentation regarding json data in body seems lacking

[Tombstone2K]

Hey,

While making my ESP32 project I really struggled with implementing proper json handling in my web server.

I finally found the solution in an old issue opened in 2017 -> https://github.com/me-no-dev/ESPAsyncWebServer/issues/123

Specifically, https://github.com/me-no-dev/ESPAsyncWebServer/issues/123#issuecomment-317796872 and https://github.com/me-no-dev/ESPAsyncWebServer/issues/123#issuecomment-1479114842

I did realize that https://github.com/me-no-dev/ESPAsyncWebServer#json-body-handling-with-arduinojson exists later, but it wasn't apparent earlier on.

In my opinion the documentation could use some upgrades for this function.

Regards