[Atlas] Clarify where to create API key for Atlas OpsGenie integration

Description

Hey docs team,

I was preparing a customer demo to showcase the integration of Atlas and OpsGenie and I was struggling to make it work. 

You can find screenshots and a more detailed description in slack:

https://mongodb.slack.com/archives/C1QM2S60K/p1623825423184400

I assume that other people might run into the same issue  Does it make sense to get a more detailed description for our docs?

Hauke

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)