[docs] server middleware

[gabrielcsapo]

This used to exist in https://ember-cli.com/user-guide/#developing-on-a-subpath but currently doesn't exist on the new docs anymore.

https://cli.emberjs.com/release/basic-use/folder-layout/ also doesn't mention the server folder as being something that is optional part of creating an app or addon.

[rwjblue]

Thank you for reporting!