[Internal]: ECH documentation gap around deployment types

### Description

At the moment, it looks like we don´t have comprehensive public documentation on how the number of Kibana UI and Background nodes provisioned is impacted by customers selecting a given memory configuration.

This [adhoc conversation](https://elastic.slack.com/archives/CHSSGF015/p1750219159054909?thread_ts=1750209423.654039&cid=CHSSGF015) and the following [KB article](https://support.elastic.dev/knowledge/view/fd4fb8f5) attempt to explain this to an internal audience.

### Resources

This [adhoc conversation](https://elastic.slack.com/archives/CHSSGF015/p1750219159054909?thread_ts=1750209423.654039&cid=CHSSGF015) and the following [KB article](https://support.elastic.dev/knowledge/view/fd4fb8f5) attempt to explain this to an internal audience.

You can also reach out to @pmuellr or @mikecote

### Which documentation set does this change impact?

Elastic Cloud Hosted only

### Feature differences

( only one selected - ECH )

### What release is this request related to?

N/A

### Serverless release

( only one selected - ECH )

### Collaboration model

The documentation team

### Point of contact.

You can also reach out to @pmuellr or @mikecote

**Stakeholders:**
@heespi @rudolf @daviddetorres