docs: Add rnaseq-nf tutorial

[christopher-hakkaart]

• Add a rnaseq-nf tutorial to act as a prerequisite for other guides and tutorials that reference it heavily.

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[christopher-hakkaart]

@bentsherman - This is what I was thinking for a precursor for many of the guides/tutorials in development. I've tried to keep it reasonably high level, but enough to explain what runs, and where it can be found.

I was thinking about linking to the specific files in the repo, i.e., when referring to the entry workflow or module, link to those in GitHub.

Let me know what you think.