Docs around middleware only apply to ember-cli-fastboot (?)

The [Request Body](https://www.ember-fastboot.com/docs/user-guide#request-body) section talks about using an in-app-addon to add middleware to your app. This works great with `ember-cli-fastboot` however it does not work at all if you are deploying to production with the (recommended) `Fastboot App Server`.

This section should be updated to reflect the different approaches needed for different deployment strategies. Not sure if an in-repo-addon can be used somehow with `Fastboot App Server`, though I kind of doubt it.