Refine ECE retention policies for logging and metrics data

[eedugon]

On #3788, @rseldner made an important change to the doc as starting in ECE 3.8, the system is using data streams for logs and metrics towards logging-and-metrics system deployment.

This issue raises some extra improvements we could add to have a clearer procedure:

2. Create a new composable index template that matches the data view (formerly index pattern) for the data stream you want to customize the lifecycle for.

a. We should add a link to the instructions on how to create the composable index template
b. we should identify the data streams that ECE is going to create by default if we tell the user to choose one.

3. Specify a custom lifecycle policy in the index template settings.

Link to the creation of lifecycle policies would be welcome here.

4. Choose a higher priority for the template so the specified lifecycle policy will be used instead of the default.

We should identify the default template names so the user can check the priority in order to create a higher one.

My suggestion is to ensure the doc also gives visibility of how the system setup looks like by default, so the user will be able to apply the needed changes.