Add docs for openapi-to-k6 project

[heitortsergent]

We have a new CLI tool to convert an OpenAPI schema to a TypeScript client, which people can then use in their k6 test scripts.

https://github.com/grafana/openapi-to-k6/

We should add a page explaining the basics of the tool and how to use it, and a testing guide showing an example of how to use it with an OpenAPI schema. This blog post could serve as a starting point for the testing guide.