Document formal script syntax

[bentsherman]

Now that the docs are in better shape, we can begin work towards a formal description of the Nextflow language, as well as prepare the docs and tests for the strict syntax that will be introduced by #4613 and #4744 .

Refer to the changes to snippets and tests for examples of strict syntax:

• replace implicit it closure parameter with explicit parameter
• use quotes for process env input/output name
• move top-level statements into process / function / workflow definitions

Currently the syntax description is split across several pages:

• Scripts
• Processes
• Workflows
• Modules

It would be nice to have a single page that describes the entire language. At the same time, I don't want to move these pages around much more, and they are too hefty individually to combine them.

Since these pages cover a lot of other topics like APIs and runtime behavior, maybe it would be better to add a new page e.g. "Reference > Syntax" which gives a top-down syntax description.

Looking to the Groovy docs as an example, I feel like the Scripts page could be expanded to also describe process/workflow/include syntax. It just feels weird to mention them here when there are already dedicated pages.