Generate a docs for properties that are a simple reference

[swallez]

Open API 3.0 doesn't allow attaching docs and additional properties to a $ref type definition. Because of this (and this was a forgotten TODO) many property descriptions were dropped in the OpenAPI output.

This PR fixes this by using a common/recommended workaround consisting in using a single allOf inline schema around the reference.

Fixes #5082.

@szabosteve can you verify that the doc generation pipeline correctly picks up this change?

Example (taken from the diff):

"chunking_settings": {
    "$ref": "#/components/schemas/inference._types.InferenceChunkingSettings"
}

Becomes:

"chunking_settings": {
    "externalDocs": {
      "url": "https://www.elastic.co/docs/explore-analyze/elastic-inference/inference-api#infer-chunking-config"
    },
    "description": "The chunking configuration object.",
    "allOf": [
     {
        "$ref": "#/components/schemas/inference._types.InferenceChunkingSettings"
      }
    ]
}

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[szabosteve]

Thank you so much for this fix, @swallez! I asked the folks responsible for the docs generation process to verify if they can pick this change up correctly. I'll get back to you as soon as I have an answer.