Add v1alpha2 migration doc

docs/migrations/v1alpha1-to-v1alpha2.md

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+# Cluster API Operator v1alpha1 API compared to v1alpha2 API
+
+This document provides an overview over relevant changes between Cluster API Operator API v1alpha1 and v1alpha2 for consumers of our Go API.
+
+## Changes by Kind
+
+The changes below affect all v1alpha1 provider kinds: `CoreProvider`, `ControlPlaneProvider`, `BootstrapPrivider` and `InfrastructureProvider`.
+
+### API Changes
+
+This section describes changes that were introduced in v1alpha2 API and how to update your templates to the new version.
+
+#### ImageMeta -> imageURL conversion
+
+In v1alpha1 we use ImageMeta object that consists of 3 parts:
+
+- Repository (optional string): image registry (e.g., "example.com/repo")
+- Name (optional string): image name (e.g., "provider-image")
+- Tag (optional string): image tag (e.g., "v1.0.0")
+
+In v1alpha2 it is just a string, which represents the URL, e.g. `example.com/repo/image-name:v1.0.0`.
+
+Example:
+
+v1alpha1
+```yaml
+spec:
+ deployment:
+   containers:
+   - name: manager
+     image:
+       repository: "example.com/repo"
+       name: "image-name"
+       tag: "v1.0.0"
+```
+
+v1alpha2
+```yaml
+spec:
+ deployment:
+   containers:
+   - name: manager
+     imageURL: "example.com/repo/image-name:v1.0.0"
+```
+
+#### secretName/secretNamespace -> configSecret conversion
+
+In v1alpha1 we have 2 separate top-level fields to point to a config secret: `secretName` and `secretNamespace`. In v1alpha2 we reworked them into an object `configSecret` that has 2 fields: `name` and `namespace`.
+
+Example:
+
+v1alpha1
+```yaml
+spec:
+ secretName: azure-variables
+ secretNamespace: capz-system
+```
+
+v1alpha2
+```yaml
+spec:
+ configSecret:
+   name: azure-variables
+   namespace: capz-system
+```