Document how to Track deployments of an external deployment tool - FluxCD ver.

Problem

We have a documentation about Tracking deployments of an external deployment tool. It currently has ArgoCD example, but we should have FluxCD example as well.

Proposal

Add an example of how to setup FluxCD notification https://fluxcd.io/flux/components/notification/ to report back the deployment status to GitLab Deployment API. So that users can unleash the other GitLab features such as:

• See when an merge request has been deployed, and to which environment.
• Filter merge requests by environment or deployment date.
• DevOps Research and Assessment (DORA) metrics.
• View environments and deployments.
• Track newly included merge requests per deployment.