Story (Required)

As a user of Argo CD operator, I want documentation on diffing customization

Background (Required)

Documentation on diffing customization is present in upstream Argo CD (https://argo-cd.readthedocs.io/en/stable/user-guide/diffing/#system-level-configuration) , but not the operator.

Resource Customization (with subkeys) feature was added in GitOps operator 1.7 and ArgoCD Operator release 0.5

Approach (Required)

Use current upstream documentation at https://argo-cd.readthedocs.io/en/stable/user-guide/diffing/#system-level-configuration 

Acceptance Criteria (Mandatory)

• Create and merge a PR to ArgoCD operator to add this documentation.