Adds OpenTelemetry output description

[olegbespalov]

What?

This PR adds the first version of the opentelemetry output description.

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.

Related PR(s)/Issue(s)

The following PRs should be merged before merging this PR:

• https://github.com/grafana/xk6-output-opentelemetry/pull/14
• https://github.com/grafana/xk6-output-opentelemetry/pull/15
• https://github.com/grafana/xk6-output-opentelemetry/pull/16