Create page for setting up environments on HPC systems

[finsberg]

We should add a dedicated section on how to setup software on HPC system. For example, how to set up fenicsx on ex3 (or saga). At least we need to provide a recipe that is easy to apply for most users. I have found that conda works quite well in most cases even though you might get better performace by installing the software from source (e.g using Spack)