added some documentation

operations/steps_to_update_experimental_release.txt

+Steps to update the current experimental release with no major upgrades (i.e. install it in the same Spack instance as before - respective CI/CD env variable is ${site}_INSTALLATION_ROOT_{env})
+
+1) Back up the Lab's experimental kernel configuration in both CSCS and JSC prod
+cp -r /srv/jupyterlab_kernels/prod/experimental /srv/backup/experimental_{date}
+2) Merge the master branch of the ebrains-spack-builds repo to the experimental_rel branch.
+Make sure that the master branch has uncommented in the spack.yaml file only those packages that have been built successfully.
+3) Start the scheduled pipeline with description "deploy experimental release in the dev CSCS cluster"
+4) Check if the pipeline executed successfully
+5) On Friday morning (not every Friday, could be when we have important or numerous changes) run the scheduled pipeline with description "deploy experimental release in prod environments".
+6) Only one job will continue (due to credentials mismatch in the gitlab runner)
+7) Check the result of the job. If it is successful retry the other job that failed previously. If it is not, restore the experimental kernel's configuration from step #1 in its original location.