docs: new structure for backend system docs

Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Co-authored-by: Johan Haals <johan.haals@gmail.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

docs/backend-system/index.md

@@ -0,0 +1,15 @@
+---
+id: index
+title: The Backend System
+sidebar_label: Overview
+# prettier-ignore
+description: The backend system
+---
+
+> **DISCLAIMER: The new backend system is under active development and is not considered stable**
+
+## Status
+
+The new backend system is under active development, and only a small number of plugins have been migrated so far. It is possible to try it out, but it is not recommended to use this new system in production yet.
+
+You can find an example backend setup in [the backend-next package](https://github.com/backstage/backstage/tree/master/packages/backend-next).