Update addon implementation

Both the [walkthrough](https://github.com/kubernetes-sigs/kubebuilder-declarative-pattern/tree/master/docs/addon/walkthrough) found in this repo, and the [implementation](https://github.com/kubernetes-sigs/kubebuilder/tree/master/plugins/addon) found in https://github.com/kubernetes-sigs/kubebuilder are out of date. Addon should be implemented as a kubebuilder plugin that builds over the base plugin functionality to replace the required files and create the new ones.

Wether this plugin should be implemented in this repo or in https://github.com/kubernetes-sigs/kubebuilder is still an open question. I can implement the addon plugin if some member of this repo specifies the steps requires to update a project built with a newer version of kubebuilder, either the last release (2.3.1) or building it from master. In any case, also provide the output of `kubebuilder version`.

Start by reading docs/addon/walkthrough in this repository and comparing it with the addon implementation in kubebuilder/plugins/addon. Clarify whether the plugin belongs here or in kubebuilder, then document the required steps for updating a project with kubebuilder 2.3.1 or master and include the output of kubebuilder version.

Written by the indexing model from the issue text.