Draft: Add `site-config` directory

These changes aim to disentangle software stack config and site-specific definitions and are based on the ebrains-23-02 branch. Several other changes were done outside the actual include lines need some discussion first:

• Sorted spack.yaml alphabetically per block of packages: jupyter, collab, software tools, workflows
• Commented out some packages that have broken dependencies or have pinned requirements in ancient versions. We should discuss if dependencies should always come in open ranges depends_on("py-pandas@1.0.5:") or with a bounded when parameter (e.g. depends_on("py-pandas@1.0.5", when="@1.2.3")) to not block ecosystem evolution.
• some fixup has been done in the release branch, which may or may not need to be ported to the main branch.
• move the Jupyter related specs to a specific site-config for the lab.ebrains.eu containers? Packages with dependencies would still pull those in, but different version constraints could be applied
• py-pybind11 is available in Spack upstream (maybe only when spack@0.19:)
• currently the MR uses $SYSTEMNAME as variable (here) for choosing the site-config