Remove dedicated page on cluster Barndard

- Architecture and hardware is well documented on hardware overview page
- Documentation of Slurm, Workspaces and Filesystems are not special to
  Barnard; thus, users should read the corresponding pages
- Currently, we do not have a special topic that needs to covered on a
  separate Barnard page
- Reduce complexity of documentation
- Fix links; Link to subsection "Barnard" in hardware overview page