stdlib and structlog on separate URLs

I recently advised a developer to add `ecs_logging.StructlogFormatter()` to our project. However he managed to use the stdlib code because they are on same page.

I propose that stdlib setup and structlog setup should be separate pages in the documentation to avoid this confusion.

https://www.elastic.co/guide/en/ecs-logging/python/current/installation.html#structlog

Also thank you for your great products!