Doc needed: Why doesn't k6 have a plug in ecosystem

[MattDodsonEnglish]

People commonly ask why k6 doesn't have a plug-in system. "Extensions are nice," they say, "but why should I have to compile my own binary?"

Now that the extensions docs have an "Explanations" sidebar, we have a great place to explain the answer to that question. Most of the information needed for this article is likely in this discussion: https://github.com/grafana/k6/issues/1353 .

CC @javaducky