✨ Auto-generate OpenAPI servers from root_path (#1596)

✨ Auto-generate OpenAPI servers from root_path (#1596)

* root_path included in servers object instead of path prefix

* ♻️ Refactor implementation of auto-including root_path in OpenAPI servers

* 📝 Update docs and examples for Behind a Proxy, including servers

* 📝 Update Extending OpenAPI as openapi_prefix is no longer needed

* ✅ Add extra tests for root_path in servers and root_path_in_servers=False

* 🍱 Update security docs images with relative token URL

* 📝 Update security docs with relative token URL

* 📝 Update example sources with relative token URLs

* ✅ Update tests with relative tokens

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>