Add xk6-zeromq extension

[luissimas]

What?

This PR adds the xk6-zeromq extension to the list of extensions.

Checklist

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

• [X] I have made my changes in the docs/sources/next folder of the documentation.
• [X] I have reflected my changes in the docs/sources/v0.50.x folder of the documentation.
• [X] I have reflected my changes in the relevant folders of the two previous k6 versions of the documentation (if still applicable to previous versions).

• [X] I have made my changes in the docs/sources/next folder of the documentation.

[CLAassistant]

All committers have signed the CLA.