Link to cloud service operation KPI

From this comment

https://hifis.net/doc/cloud-service-kpi/ should imo be linked to both:

• cloud/provider (to tell providers what kind of data we will need from them)
• cloud/Helmholtz_cloud (to tell stakeholders what we are currently doing in the scope of Helmholtz Cloud Services, including monitoring and KPI recording)

Closes #213 (closed)