[DOCS] - Document how to extend the available list of addons

[julianxhokaxhiu]

Issue Type

Chore

Description

The Context:
As an end user, I would like to be informed how can I extend the currently available list of add-ons provided through the app-of-apps.yaml, with a custom one I may want to integrate.

The Solution:
Provide detailed documentation, containing a list of steps to follow to extend the installation procedure and ensure it considers my custom addon as well. Ideally, an example would also be nice to have as a kick-starter template.

The Alternative:
If not possible to extend the current list of addons using ArgoCD, another way would be to document that it is possible to deploy resources on Kubernetes the usual classical way ( via Helm/Kubectl ) but they won't be maintained using the GitOps approach.