Add example for using global variables

[heitortsergent]

What?

Add a page explaining how to use global variables in k6.

Checklist

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the npm start command locally and verified that the changes look good.

• I have made my changes in the docs/sources/k6/next folder of the documentation.
• I have reflected my changes in the docs/sources/k6/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the relevant folders of the two previous k6 versions of the documentation (if still applicable to previous versions).

[heitortsergent]

@tom-miseur @oleiade this is my draft for adding the example of how to use global variables. If you could please take a look and let me know what you think, I could definitely use some help with the explanations of each option. 🙇

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