[Alerting]: Add example to rate aggregations page

### Description

We decided to pull the example from https://github.com/elastic/observability-docs/pull/3676 because it needed more context.

Here's the content that we pulled. (I did not delete the image in case we want to use it when we develop the example.) We may choose to use a different example, just adding the content here in case we want to use it as a starting point.

```
[discrete]
[[network-rate-example]]
== Network rate example

The rule definition in this example creates an alert if the rate of bytes received by a host on all network interfaces exceeds 10 bit/s:

[role="screenshot"]
image::images/alerts-rate-aggregation.png[Rule configuration that shows Aggregation A set to RATE host.network.ingress.bytes with a threshold of IS ABOVE 10 FOR THE LAST 1 minute]

```

### Resources

Related PR: https://github.com/elastic/observability-docs/pull/3676

### Which documentation set does this change impact?

Stateful and Serverless

### Feature differences

n/a

### What release is this request related to?

N/A

### Collaboration model

The documentation team

### Point of contact.

**Main contact:** @maryam-saeidi

**Stakeholders:**