[GSSI] Add section on how to use EESSI

[stevenvdb]

Add a section showing how users can setup EESSI on the VSC clusters. Placeholder is source/compute/software/using_software/using_eessi.rst in the restructure_and_update_software_section branch. VUB already has a section in their documentation which could be used for inspiration, see https://hpc.vub.be/docs/software/modules/#european-environment-for-scientific-software-installations (I could not add @laraPPr as an assignee).