doc: add migration guide for helm new layout - Not visible in website (#3342)

**Description of the change:** - Add doc with the steps required that need is followed by the users migrate their project for the new layout. IMPORTANT: It is not visible on the website. The follow-up PR #3326 will make it visible. **Motivation for the change:** SDK is in a process to be integrated with KB which means that its project layouts will be aligned. More info : [Integrating Kubebuilder and Operator SDK](https://github.com/kubernetes-sigs/kubebuilder/blob/master/designs/integrating-kubebuilder-and-osdk.md). As part of this process, we are creating the Helm plugin. See PR #3295. Also, we are updating the steps in the doc in the PR #3326 and here we are adding a new doc which will help users to migrate their projects.

Author: camilamacedo86

website/content/en/docs/helm/migration.md

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+
+This guide walks through an example of migrating a simple nginx-operator which was built by following the [legacy quick-start][quickstart] to the new layout.
+
+## Overview
+
+The motivations for the new layout are related to bringing more flexibility to users and 
+part of the process to [Integrating Kubebuilder and Operator SDK][integration-doc].
+
+### What was changed
+ 
+The `deploy` directory was replaced with the `config` directory including a new layout of Kubernetes manifests files
+- CRD's manifests in `deploy/crds/` are now in `config/crd/bases`
+- CR's manifests in `deploy/crds/` are now in `config/samples`
+- Controller manifest `deploy/operator.yaml` was replaced for `config/manager/manager.yaml` 
+- RBCA's manifests in `deploy` are in `config/rbac/`
+
+The `build/Dockerfile` directory was replaced by the `Dockerfile` in the root directory
+
+### What is new
+
+- Now users are able to use [kustomize][kustomize] in the configurations files
+- PROJECT file in the root directory has all information about the project
+- Users are able to customize commands for your own projects via the Makefile which is added on the root directory
+
+## How to migrate
+
+The easy migration path is to create a new project from the scratch and let the tool scaffold the files properly and then, 
+just replace with your customizations and implementations. Following an example. 
+ 
+### Creating a new project
+
+Let's create the same project but with the Helm plugin:
+
+```sh
+$ mkdir nginx-operator
+$ cd nginx-operator
+$ operator-sdk init --plugins=helm.operator-sdk.io/v1 --domain=com --group=example --version=v1alpha1 --kind=Nginx
+```
+
+**Note** Ensure that you use the same values for the flags to recreate the same Helm Chart and API's. If you have
+more than one chart or API's you can add them via `operator-sdk create api` command. For further information check the [quick-start][quickstart-new]. 
+ 
+### Replacing the content
+
+- Update the CR manifests in `config/samples` with the values of the CR's in your old project which are in `deploy/crds/`
+- Check if you have customizations options in the `watch.yaml` file of your previous project and then, update the new `watch.yaml` file with the same ones
+- Ensure that all roles configured in the `/deploy/roles.yaml` will be applied in the new project in the file `config/rbac/role.yaml`
+- If you have customizations in your `helm-charts` then, apply them in the new `helm-charts`. Note that this directory was not changed at all.
+ 
+### Checking the changes
+
+Now, follow the steps in the section [Build and run the operator][build-run-quick] to verify your project is running. 
+
+<!-- todo: update the following link to /docs/helm/legacy/quickstart when the PR #3326 get merged -->
+[quickstart]: /docs/helm/quickstart
+[quickstart-new]: /docs/helm/quickstart
+[integration-doc]: https://github.com/kubernetes-sigs/kubebuilder/blob/master/designs/integrating-kubebuilder-and-osdk.md
+[build-run-quick]: /docs/helm/quickstart#build-and-run-the-operator
+[kustomize]: https://github.com/kubernetes-sigs/kustomize