Documentation for stub running

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Is your feature request related to a problem? Please describe

In the cases where tests absolutely require too large files, there is functionality for running as stub. However this needs more documentation (as suggested by @drpatelh).

Describe the solution you'd like

It would be nice with a short tutorial, including clear guidelines for when stub-running is ok and when it is not. Additionally it could - on a more long term - be nice if this was included as nf-core scripting functionality, but I think for now just some text along with example modules would suffice.

Describe alternatives you've considered

It's possible to make stub-running already once one knows which modules to look at. Phyloflash by @ramprasadn is a good example.

Additional context

Maybe it'd be a good idea for the upcoming hackaton?