Add docs for jslib http-instrumentation-tempo

[mstoykov]

What?

Documentation for https://github.com/grafana/jslib.k6.io/pull/121

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).

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

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