[Request]: Document how to route APM Traces, Logs, Metrics to a dedicated namespace

### Description

Users often request the ability to route the data (traces, metrics, logs) sent from a specific group of agents to a dedicated custom namespace.
The reason can be to separate data from different teams or environments.

This can be achieved in 2 ways:
1. Create a dedicated APM Server or Integration with a specific namespace. All data will land in that namespace
2. Create `<...>@custom` ingest pipelines and use the `reroute` processor. This flow is available only in 8.12+ https://support.elastic.dev/knowledge/view/6a7e483b

It would be interesting to cover few examples like:
- How to create a namespace based on the environment
- I would warn about data streams explosion when using this technique with the service name (E.g. using a conditional which routes only specific services to a specific namespace).

If instead we want to recommend to not do any customizations, we should warn about this in our doc with the reasons why it might be a better choice to leave defaults.

### Resources

-

### Which documentation set does this change impact?

Stateful and Serverless

### Feature differences

On Serverless, APM Server/Integration is replaced by a service which removes the option (1) from the picture.

### What release is this request related to?

8.12

### Collaboration model

Other (please describe below)

### Point of contact.

**Main contact:** @simitt or @felixbarny ?

**Stakeholders:** @lucabelluccini

This is not an urgent item, but we often see this kind of questions from users and having an official recommendation by Elastic would help.