Create doc that explains all places one can "generate load from"

[markjmeier]

As the title states, we currently have quite a few ways people can generate load, for different use cases. We often get questions about testing "on prem" when most people are looking to generate load on prem. We should document what is available so users can choose the right solution without needing to talk to us:

Public available endpoints:

• Grafana cloud k6 public load gens

Interval/private endpoints:

• Static IPs
• k6 OSS -o cloud command
• k6 operator
• Private load zones