docs: update migration docs with info on apiVersion field

docs/reference/project-config.md

@@ -20,6 +20,9 @@ Please refer to those for more details on provider configuration.
 The values in the schema below are the default values.
 
 ```yaml
+# Schema version of the config.
+apiVersion: garden.io/v1
+
 # Indicate what kind of config this is.
 kind: Project
 
@@ -196,6 +199,14 @@ variables: {}
 ## Configuration Keys
 
 
+### `apiVersion`
+
+Schema version of the config.
+
+| Type     | Allowed Values                 | Default          | Required |
+| -------- | ------------------------------ | ---------------- | -------- |
+| `string` | "garden.io/v0", "garden.io/v1" | `"garden.io/v1"` | Yes      |
+
 ### `kind`
 
 Indicate what kind of config this is.

docs/tutorials/migrating-to-bonsai.md

@@ -10,6 +10,7 @@ On the other hand, the optional new configuration format is easier to read and w
 
 Here is the list of breaking changes when upgrading your `garden` CLI tool. This lets you use your old Module-style configuration files with minimal changes.
 
+- `apiVersion` is now mandatory in project config files for new action-Kind configs. It's recommended to set the value to `garden.io/v1` for new projects. For backwards compatibility, garden will assume the apiVersion `garden.io/v0`.
 - `cert-manager` integration has been removed. New documentation has been created in the [ext dns and cert manager example](../../examples/cert-manager-ext-dns)
 - `dev-mode` has been renamed to `sync`, both in the configuration as well as on the CLI
 - `garden delete` has been renamed to `garden cleanup`