[Atlas] Documentation of Customer Key Management for GCP points Azure Key Vault

Description

The following page covers customer key management of Atlas Clusters through GCP KMS.

https://docs.atlas.mongodb.com/security-gcp-kms/ 

However, it points out AKV (Azure Key Vault) in the top of the page:

"This page covers configuring customer key management using AKV on your Atlas project."

Scope of changes

Impact to Other Docs

MVP (Work and Date)

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