[DOCS] Adds EIS reference docs

[szabosteve]

Overview

This PR drafts reference docs for the Elastic Inference Service.

TO DO

• Review and amend task_settings
• Add chat_completion example

Post-merge TO DO

• Link to the conceptual docs (tracking issue: https://github.com/elastic/search-docs-team/issues/245)
• Expand description with limitation info

Preview

EIS reference

[github-actions]

Documentation preview:

• ✨ Changed pages

[jonathan-buttner]

Here's an example of creating an EIS endpoint for chat completion:

PUT http://localhost:9200/_inference/chat_completion/my-endpoint
{
    "service": "elastic",
    "service_settings": {
        "model_id": "model-1"
    }
}

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x