[8.x] [DOCS] Documents trained model auto-scaling (backport #2795)

[mergify]

Overview

This PR:

• creates a new page about model autoscaling,
• provides tables for the autoscaling behavior in Cloud, (the content for Serverless will be part of another PR)
• gives instructions on how to optimize a model for search/ingest by using the API,
• adds link to ELSER, E5, and Deploy trained model pages that points to the autoscaling page,
• removes the corresponding ELSER limitation,
• reviews the text on Deploy the model in your cluster page,
• updates the screenshot on the Deploy the model in your cluster page.

Preview

• Trained model autoscaling

Remarks

elastic/elasticsearch#114038 removed a link to adaptive allocations docs from the Semantic text tutorial to avoid failing docs builds. The link needs to be updated and put back in the tutorial after this PR is merged.

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This is an automatic backport of pull request #2795 done by [Mergify](https://mergify.com).

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