Document automatic stackable version (#433)

* Document automatic stackable version

* Update modules/concepts/pages/product_image_selection.adoc

Co-authored-by: Andrew Kenworthy <andrew.kenworthy@stackable.de>

---------

Co-authored-by: Andrew Kenworthy <andrew.kenworthy@stackable.de>

Author: sbernauer

modules/concepts/pages/product_image_selection.adoc

@@ -14,20 +14,28 @@ There are multiple ways to specify the image used:
 If your Kubernetes cluster has internet access, the easiest way is to use the publicly available Images from the https://docker.stackable.tech/[Image registry hosted by Stackable].
 If the Kubernetes cluster does not have internet access, a xref:_custom_docker_registry[] or xref:_custom_images[] can be used.
 
-Currently, you need to specify two versions:
+Currently, you need to specify the product version. This can be found on the xref:operators:supported_versions.adoc[list of supported product versions] or on the website of the product itself.
+This requirement might be relaxed in the future, as every platform release will ship wth a recommended product versions, which will be used by default.
+
+Additionally you can specify the Stackable version: As we need to make changes to the Images from time to time (e.g. security updates), we also have to version them using the Stackable version. An image gets released for every version of the SDP.
+There are two variants you can choose from:
+
+1. Fixed version, e.g. `23.7.0`. This image will never change.
+2. Release line, e.g. `23.7`. This will be a floating tag pointing to the latest patch release of the SDP release line. It will contain the latest security patches, but will also change over time.
+
+If you don't specify the Stackable version, the operator will use its own version, e.g. `23.7.0`.
+When using a nightly operator or a `pr` version, it will use the nightly `0.0.0-dev` image.
 
-1. **Product version:** The version of the product, as it e.g. can be found on the xref:operators:supported_versions.adoc[list of supported product versions] or on the website of the product itself.
-2. **Stackable version:** As we need to make changes to the Images from time to time (e.g. security updates), we also have to version them using the Stackable version.
 All the available images (with their product and stackable version) can be found in our https://repo.stackable.tech/#browse/browse:docker:v2%2Fstackable[docker repository].
 
-The versions need to be specified on Stackable products as follows:
+The versions need to be specified as follows:
 
 [source,yaml]
 ----
 spec:
   image:
     productVersion: 3.3.1
-    stackableVersion: 0.1.0
+    # stackableVersion: 23.7.0 # optional, or e.g. 23.7
 ----
 
 == Custom docker registry
@@ -42,11 +50,11 @@ Afterwards you can use the following snippet to configure your custom docker rep
 spec:
   image:
     productVersion: 3.3.1
-    stackableVersion: 0.1.0
+    stackableVersion: 23.7.0
     repo: my.corp/myteam/stackable
 ----
 
-This will change the image from the default Stackable repository `docker.stackable.tech/stackable/kafka:3.3.1-stackable0.1.0` to `my.corp/myteam/stackable/kafka:3.3.1-stackable0.1.0`.
+This will change the image from the default Stackable repository `docker.stackable.tech/stackable/kafka:3.3.1-stackable23.7.0` to `my.corp/myteam/stackable/kafka:3.3.1-stackable23.7.0`.
 
 == Custom images