doc: Adds demo links to migration examples in README files (#3301)

Author: EspenAlbert

examples/migrate_cluster_to_advanced_cluster/basic/README.md

@@ -1,5 +1,8 @@
 # Basic Migration from `mongodbatlas_cluster` to `mongodbatlas_advanced_cluster`
 
+**Note**: See [Terraform Cluster to Advanced Cluster Resource Migration](https://www.youtube.com/watch?v=WiYok6_JfI8) for a recorded demo of this example.
+
+
 This example demonstrates how to migrate a `mongodbatlas_cluster` resource to `mongodbatlas_advanced_cluster` (see alternatives, and more details in the [cluster to advanced cluster migration guide](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/cluster-to-advanced-cluster-migration-guide)).
 In this example we use specific files, but the same approach can be applied to any configuration file with `mongodbatlas_cluster` resource(s).
 The main steps are:

examples/migrate_cluster_to_advanced_cluster/module_maintainer/README.md

@@ -1,5 +1,8 @@
 # Module Maintainer - `mongodbatlas_cluster` to `mongodbatlas_advanced_cluster`
 
+**Note**: See [Terraform Cluster to Advanced Cluster Migration Module Maintainer Perspective](https://www.youtube.com/watch?v=f-B9sK8n2rY) for a recorded demo of this example.
+
+
 If you own and maintain modules to manage your Terraform resources, the purpose of this example is to demonstrate how a Terraform module definition can migrate from `mongodbatlas_cluster` to `mongodbatlas_advanced_cluster` while minimizing impact to its clients. The [other module example](https://github.com/mongodb/terraform-provider-mongodbatlas/tree/master/examples/migrate_cluster_to_advanced_cluster/module_user) explains the same process from the module user point of view.
 
 The example contains three module versions which represent the three steps of the migration:

examples/migrate_cluster_to_advanced_cluster/module_user/README.md

@@ -1,5 +1,8 @@
 # Module User - `mongodbatlas_cluster` to `mongodbatlas_advanced_cluster`
 
+**Note**: See [Terraform Cluster to Advanced Cluster Migration Module User Perspective](https://www.youtube.com/watch?v=5npTiwMcCFs) for a recorded demo of this example.
+
+
 The purpose of this example is to demonstrate the experience of adopting a new version of a terraform module definition which internally migrated from `mongodbatlas_cluster` to `mongodbatlas_advanced_cluster`.
 Each module call represent a step on the migration path.
 The example focus on the call of the module rather than the module implementation itself (see the [module maintainer README.md](../module_maintainer/README.md) for the implementation details).