Document squashfs module requirement for Apptainer

[FredDroest]

Added information about loading the squashfs module for container image handling in Apptainer, required for using certain nf-core pipelines on the LRZ HPC cluster.

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name: New Config
about: A new cluster config

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• If your PR is a work in progress, include [WIP] in its title
• Your PR targets the master branch
• You've included links to relevant issues, if any

Steps for adding a new config profile:

• Add your custom config file to the conf/ directory
• Add your documentation file to the docs/ directory
• Add your custom profile to the nfcore_custom.config file in the top-level directory
• Add your profile name to the profile: scope in .github/workflows/main.yml
• OPTIONAL: Add your custom profile path and GitHub user name to .github/CODEOWNERS (**/<custom-profile>** @<github-username>)

[nschan]

This is a good point. The squashfs module is loaded automatically when loading the apptainer module, maybe it would then be more convenient to load the whole apptainer+squashfs thing via module instead?