docs: Clarify distinction between modules and script inclusion

[bentsherman]

When DSL2 was introduced, "modules" referred to both "a process in a script" and "any script included by another script". Over time, I ended up emphasizing the latter definition in the docs.

However, with the emergence of registry modules, subworkflows, and meta-pipelines, the meaning of "module" has become muddled. This shows up in several places:

• the disconnect between Nextflow and nf-core's definition of a module (nf-core subworkflow != nf-core module)
• the explanation of local vs registry modules in the docs

I think it has become clear that a "module" is really just a "process in a script". A workflow in a script is not a module.

This PR clarifies the definition of "module", separating it from the more general concept of "script inclusion". It aligns the use of "module" with the module registry, module CLI, and nf-core convention.

Related: #6650 , #7213

TODO:

• introduce script inclusion in the "Scripts" page instead of the modules overview?

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[christopher-hakkaart]

I like the direction to limit the definition.

A module is a script that contains a single process definition.

With this definition, it's really just the main.nf with only one process definitions. Where does this leave associated files? E.g., meta.yml. Do we refer to those as module files?

[christopher-hakkaart]

We should also look at the training to make sure this is echoed there

[bentsherman]

I notified the training team about this PR, they will double-check their material

[bentsherman]

@christopher-hakkaart I think this PR is ready now

[christopher-hakkaart]

Looks good!