[Internal]: Installing Knowledge Base might trigger auto-scaling of ML nodes or lead to errors if undersized ES ML nodes

### Description

Hello,

We should document the fact that installing Knowledge Base in the Search or O11y AI Assistant can incur in extra costs as docs might not be indexed with the same model as before - and it might upscale of ML nodes.

If the ML nodes are undersized, we should have a short troubleshooting section in the docs for the users on how to identify and resolve this.

### Resources

I've been informed by this by @emma-raffenne and she told me to loop in Arturo Liduena.

I do not have a lot of background but we can sync via Slack if necessary.

### Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

### Feature differences

IDK

### What release is this request related to?

N/A

### Serverless release

IDK

### Collaboration model

The engineering team

### Point of contact.

**Main contact:** @arturoliduena

**Stakeholders:** @emma-raffenne and @mdbirnstiehl