docs: Rename to debian-upgrade.rst file

Refs. swh/infra/sysadm-environment#5556

docs/sysadm/data-silos/index.rst

@@ -9,4 +9,4 @@ Data silos
    kafka/index
    elasticsearch/index
    winery/index
-   rancher/debian
+   rancher/index

docs/sysadm/data-silos/rancher/index.rst

+.. _rancher:
+
+Rancher
+=======
+
+.. toctree::
+   :titlesonly:
+
+   debian-upgrade
+
+.. todo::
+   This page is a work in progress.
+