T7253: Improve documentation about upgrades with recommendations

Improve the documentation about upgrades and rollbacks with information about how to handle settings.

content/architecture/platform-guide.md

@@ -317,6 +317,18 @@ by the [Apertis GitLab instance](https://gitlab.apertis.org/):
 
 ## Image creation
 
+Image creation is the point where a set of standard packages are combined to
+build a solution for a specific use case. This goal is accomplish thanks to
+[Debos](https://github.com/go-debos/debos), a flexible tool to configure the
+build of Debian-based operating systems. Debos uses tools like `debootstrap`
+already present in the environment and relies on virtualisation to securely do
+privileged operations without requiring root access.
+
+Additionally at this stage customizations can be applied by using overlays.
+This process allows the default content of the packages to be combined with
+custom modifications to provide the desired solution. A common case is to apply overlays to
+change some default system settings found in `/etc` such as default hostname.
+
 Ospacks and how they should be processed to generate images are defined through
 YAML files.