Add docs for versions SSoT

CONTRIBUTING.md

@@ -9,16 +9,19 @@ from [`.gitlab/README.md.template`](.gitlab/README.md.template).
 
 ## Upgrade OpenTofu versions
 
-When adding new OpenTofu versions the following places need to be updated:
-
-| File                                                         | What                                                                                      |
-| ------------------------------------------------------------ | ----------------------------------------------------------------------------------------- |
-| [`templates/full-pipeline.yml`](templates/full-pipeline.yml) | The `default` value and `options` list of the `sepc.inputs.opentofu_versions` entry.      |
-| [`.gitlab-ci.yml`](.gitlab-ci.yml)                           | The `.opentofu_versions.parallel.matrix` list and the `LATEST_OPENTOFU_VERSION` variable. |
-
-All of the above definitions have to match each other.
-We currently need to change it in multiple places, because there is not a good way to share information
-from the templates and the components pipeline defintion - at least in the features we'd like to use them.
+The file `opentofu_versions.yaml` in the root of the project acts as
+Single Source of Truth (SSoT) for the OpenTofu versions currently supported.
+The versions in the templates are rendered by the
+`./.gitlab/scripts/update-opentofu-versions.sh` script and the versions in
+the `.gitlab-ci.yml` file are referencing the data from the SSoT file by
+using a combination of CI includes and `!reference`.
+
+To update the supported OpenTofu versions perform the following steps:
+
+- Update the SSoT file with the new latest version and supported versions.
+- Run the `./.gitlab/scripts/update-opentofu-versions.sh` script.
+- Commit the changes.
+- Submit Merge Request.
 
 ## Releasing