Updates the Watsonx integration page with reranking feature (#623)

##
[Preview](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/623/solutions/search/inference-api/watsonx-inference-integration)

This PR updates the Watsonx integration page to include the new 'rerank'
feature.

Related issue: elastic/developer-docs-team#255

Author: kosabogi

solutions/search/inference-api/watsonx-inference-integration.md

@@ -33,9 +33,8 @@ You need an [IBM Cloud® Databases for Elasticsearch deployment](https://cloud.i
 
     Available task types:
 
-    * `text_embedding`.
-
-
+    * `text_embedding`,
+    * `rerank`.
 
 ## {{api-request-body-title}} [infer-service-watsonx-ai-api-request-body] 
 
@@ -50,9 +49,9 @@ You need an [IBM Cloud® Databases for Elasticsearch deployment](https://cloud.i
     `api_key`
     :   (Required, string) A valid API key of your Watsonx account. You can find your Watsonx API keys or you can create a new one [on the API keys page](https://cloud.ibm.com/iam/apikeys).
 
-        ::::{important} 
-        You need to provide the API key only once, during the {{infer}} model creation. The [Get {{infer}} API](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-inference-get) does not retrieve your API key. After creating the {{infer}} model, you cannot change the associated API key. If you want to use a different API key, delete the {{infer}} model and recreate it with the same name and the updated API key.
-        ::::
+    ::::{important} 
+    You need to provide the API key only once, during the {{infer}} model creation. The [Get {{infer}} API](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-inference-get) does not retrieve your API key. After creating the {{infer}} model, you cannot change the associated API key. If you want to use a different API key, delete the {{infer}} model and recreate it with the same name and the updated API key.
+    ::::
 
 
     `api_version`
@@ -70,13 +69,28 @@ You need an [IBM Cloud® Databases for Elasticsearch deployment](https://cloud.i
     `rate_limit`
     :   (Optional, object) By default, the `watsonxai` service sets the number of requests allowed per minute to `120`. This helps to minimize the number of rate limit errors returned from Watsonx. To modify this, set the `requests_per_minute` setting of this object in your service settings:
 
-        ```text
-        "rate_limit": {
-            "requests_per_minute": <<number_of_requests>>
-        }
-        ```
+```json
+"rate_limit": {
+    "requests_per_minute": <<number_of_requests>>
+}
+```
+
+`task_settings`
+:   (Optional, object) Settings to configure the inference task. 
+    
+    These settings are specific to the `<task_type>` you specified.
+
+::::{dropdown} `task_settings` for the `rerank` task type
+`truncate_input_tokens`
+:   (Optional, integer) Specifies the maximum number of tokens per input document before truncation.
+
+`return_documents`
+:   (Optional, boolean) Specify whether to return doc text within the results.
 
+`top_n`
+:   (Optional, integer) The number of most relevant documents to return. Defaults to the number of input documents.
 
+::::
 
 ## Watsonx AI service example [inference-example-watsonx-ai] 
 
@@ -101,4 +115,35 @@ PUT _inference/text_embedding/watsonx-embeddings
 3. The ID of your IBM Cloud project.
 4. A valid API version parameter. You can find the active version data parameters [here](https://cloud.ibm.com/apidocs/watsonx-ai#active-version-dates).
 
+The following example shows how to create an inference endpoint called `watsonx-rerank` to perform a `rerank` task type.
+
+```console
+
+PUT _inference/rerank/watsonx-rerank
+{
+    "service": "watsonxai",
+    "service_settings": {
+        "api_key": "<api_key>", <1>
+        "url": "<url>", <2>
+        "model_id": "cross-encoder/ms-marco-minilm-l-12-v2",
+        "project_id": "<project_id>", <3>
+        "api_version": "2024-05-02" <4>
+    },
+    "task_settings": {
+        "truncate_input_tokens": 50, <5>
+        "return_documents": true, <6>
+        "top_n": 3 <7>
+    }
+}
+```
+
+1. A valid Watsonx API key. You can find on the [API keys page of your account](https://cloud.ibm.com/iam/apikeys).
+2. The {{infer}} endpoint URL you created on Watsonx.
+3. The ID of your IBM Cloud project.
+4. A valid API version parameter. You can find the active version data parameters [here](https://cloud.ibm.com/apidocs/watsonx-ai#active-version-dates).
+5. The maximum number of tokens per document before truncation.
+6. Whether to return the document text in the results.
+7. The number of top relevant documents to return.
+
+