feat(terraform): add update howto (#33)

docs/content/terraform/howtos/update.md

@@ -0,0 +1,61 @@
+---
+title: Updating the module and library version
+weight: 30
+---
+
+The [avm-ptn-alz](https://aka.ms/alz/module) Terraform module, and its associated [provider](https://aka.ms/alz/provider) will be updated from time to time.
+Similarly, the policy library will also be updated with new features and bug fixes.
+Unlike previously, the module and the policy library are now decoupled, and can be updated independently.
+
+## Updating the policy library
+
+To update the policy library, you must set the [`library_references`](https://registry.terraform.io/providers/Azure/alz/latest/docs#library_references-1) configuration in the provider.
+Each provider release will have a default value, which will be the latest version of the ALZ policy library at the time of the release.
+To find the default value, you can check the [provider documentation](https://registry.terraform.io/providers/Azure/alz/latest/docs#library_references-1).
+
+To update the policy library, you can set the `library_references` configuration to the desired version:
+
+```hcl
+provider "alz" {
+  library_references = [
+    {
+      path = "platform/alz",
+      tag  = "2025.01.0"
+    }
+  ]
+}
+```
+
+Then run a terraform plan to see the changes that will be applied.
+
+### Local library
+
+If you have a local library, you may have a [`alz_library_metadata.json`](https://azure.github.io/Azure-Landing-Zones-Library/assets/metadata/) file in the root.
+If this file contains dependencies, you should update any dependent libraries here.
+
+###  Breaking changes
+
+In line with the library [versioning](https://azure.github.io/Azure-Landing-Zones-Library/versioning/) approach, there may be breaking changes on any year or month change.
+Patch releases will not contain breaking changes.
+We try to ensure that the changes are apply-able with Terraform, without manual intervention required.
+
+## Updating the module
+
+Over time, the module will be updated with additional features and bug fixes.
+If you wish to update the module, you can simply update the version in the module source:
+
+```hcl
+module "alz" {
+  source = "Azure/avm-prn-alz/azurerm"
+  version = "0.10.0"
+
+  # ... other configuration
+}
+```
+
+Then run a terraform plan to see the changes that will be applied.
+
+{{< hint type="warning" >}}
+The module may require an update to the provider.
+Ensure that the default value for [`library_references`](https://registry.terraform.io/providers/Azure/alz/latest/docs#library_references-1) hasn't changed and update it if necessary.
+{{< /hint >}}