Update CI and contributors documentation.

CI.md

-Currently CI is used to generate Docker images related to the training.
+Currently CI is used for three kind of jobs:
 
-These images are created through the Gitlab Web interface:
+- Checking all the Jupyter cells are not executed. If you follow the instructions [here](./CONTRIBUTING.md) it should already be ok, but it's checked nonetheless in CI in case pre-commit was not properly activated.
+- Generation of three Docker images:
+    * One to run the notebooks
+    * One with a Fedora environment that may be used to run hands-on.
+    * One in use for a demonstration.
+- Check all the hands-on solutions compile properly.
 
-- Go to CI/CD and choose Run Pipeline.
-- Fill a field _TAG_ and put the name of the tag to associate to the image.
-- Fill a field _UPDATE_LATEST_TAG_ if you want the _latest_ tag to be updated with the image being created here. This field is optional - if not filled the _latest_ tag won't be modified.
+Each Docker image is generated if one of two conditions are met:
+    * Dockerfile has been changed.
+    * A `git tag` has been provided.
 
+The image is generated but is pushed on the registry *only* if a `git tag` is specified.
+
+So to make the CI work you should first do something like:
+
+```shell
+git tag -a v24.03 -m "Release from March 2024""
+git push mine --tags
+```
+
+where `mine` is the name of the remote on Gitlab (replace with the name you use for your Gitlab remote, which might even be `origin` if you clone your fork directly - see [CONTRIBUTING file](./CONTRIBUTING.md)).

CONTRIBUTING.md

@@ -43,3 +43,5 @@ git push mine *your_branch_name*
 ![](Images/CreateMR.png)
 
 By default it will propose to push it toward the main repository of the formation.
+
+Please read [the CI README](./CI.md) to set up properly your fork!
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