Update process snippets to comply with strict syntax

[bentsherman]

The strict syntax is a bit more picky about the process script: section label (source):

The Nextflow language specification allows the script: label to be omitted only if there are no other sections

This PR updates process snippets throughout the docs to comply with this rule.

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[christopher-hakkaart]

Adding script: looks good to me. It does make the the """ vs ''' more noticeable in examples. What do you think about standardizing these as """ (unless single quotes are required)?

[bentsherman]

The rules around single vs double quotes:

• if you need to reference a Nextflow variable, you must use double quotes
• otherwise, if you need to reference a Bash variable, it's easier to use single quotes so that you don't have to escape the dollar sign
• otherwise, it doesn't matter

I have updated the process scripts to be more consistent, i.e. use double quotes except where it's cleaner to use single quotes