document how to deploy to a subdirectory

This should help to prevent situations as described in #2992.

Currently I did not cover how to handle incoming requests. I'm not sure what situations we need to cover as there can be different setups (HTTP Proxy, edit `config.ru`)

Currently there are many blog posts, stackoverflow questions and issues related to deploying into a subdirectory. As the mechanics changed slightly in the past not all resources are correct leaving most users confused. I think it's importent to cover this in the official docs so that users can lookup the steps for different versions.

This is only a scaffold. Let me know if you have content or copy suggestions.