manual/workflow-tips: simplify the basic workflow (& explain _why_ we put code in a module)

[tfiers]

I came upon this piece of documentation again after seeing this popular post by @bkamins: https://discourse.julialang.org/t/how-to-clear-variables-and-or-whole-work-space/10149/21?u=tfiers

("How to clear variables and/or whole work space?" → "Work in a module")

This small addendum to the Worfklow Tips section of the manual answers "but why should I bother making that module";
basically putting the gist of that discourse thread in the documentation, where more people might see it

[tfiers]

suggestions for other wording welcome ("the Main module of the REPL" might not be something accurate, e.g.)

[vtjnash]

Suggested change

	The key here is to encapsulate the code in a module.
	If it were not so, and the code was ran directly in the `Main` module of the REPL,
	you would not be able to remove methods or edit `struct` definitions, without restarting Julia.
	The key here is to encapsulate the code in a module.
	This makes the global state more isolated, so that it is not affected by prior content in the `Main` module of the REPL,
	or methods and `struct` definitions added before.
	This reduces the need to restart Julia in this workflow.

[vtjnash]

(changed to word this only in the positive, and to mention another benefit of this workflow tip)

[tfiers]

Thank you for the review and suggestion :)

The isolation from previous REPL commands is indeed another benefit.
Maybe confusing for new users that "the global state" refers to the state inside the module?

Does using a module reduce the need for restarting Julia, or (virtually) eliminate it?

I'll tag the original author of this section (4b51aef), @twadleigh, to ask them what they think.

[tfiers]

Actually this edit changes what I meant: I think that in an "efficient interactive workflow", which this guide is about, the main advantage of a module is not having to restart Julia when editing structs or removing methods.

[tfiers]

Thinking about this whole section more, I think it can be simplified further:

• why even need a separate test file? Just write it in the same file, underneath the module. Or even inside the module?
• why both the import and using lines. This is a place to be opinionated (i.e. pick using)

[tfiers]

I rewrote (simplified) the entire section.
Adding the original author @twadleigh as additional reviewer, and @KristofferC, who previously (re-)simplified this section (#30194)

Very open to new suggestions, this is a draft.

Edit: (seems like I can't add reviewers). Input welcome anyway