Add link to GitHub source code for all jslib libraries

[heitortsergent]

What?

Add an admonition at the top of all jslib libraries _index pages with a link to their GitHub repositories.

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/next folder of the documentation.
• I have reflected my changes in the docs/sources/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).

[yorugac]

This looks great, IMO! Thanks to the colored note, the links are even more obvious than I hoped initially 🙌 Thank you @heitortsergent 💛

I've left a suggestion about a couple of links, that they should be more precise: this is my opinion as a user browsing documentation 🙂

For posterity's sake, attaching how it looks now:

[heitortsergent]

Thanks for the review @yorugac. 🙇