[ML] Adding docs for the unified inference API (elastic#118696)

* Including examples

* Using js instead of json

* Adding unified docs to main page

* Adding missing description text

* Refactoring to remove unified route

* Addign back references to the _unified route

* Update docs/reference/inference/chat-completion-inference.asciidoc

Co-authored-by: István Zoltán Szabó <[email protected]>

* Address feedback

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Co-authored-by: István Zoltán Szabó <[email protected]>

Author: jonathan-buttner