Add rank eval documentation (#1316)

* updated incorrect links

Signed-off-by: ariamarble <armarble@amazon.com>

* adding documentation for rank eval api

Signed-off-by: ariamarble <armarble@amazon.com>

* made suggusted changes

Signed-off-by: ariamarble <armarble@amazon.com>

* quick syntaxt changes

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* editorial updates

Signed-off-by: ariamarble <armarble@amazon.com>

Signed-off-by: ariamarble <armarble@amazon.com>

_opensearch/rest-api/rank-eval.md

@@ -0,0 +1,115 @@
+---
+layout: default
+title: Ranking evaluation
+parent: REST API reference
+nav_order: 82
+---
+
+# Ranking evaluation
+
+The [rank]({{site.url}}{{site.baseurl}}/opensearch/supported-field-types/rank/) eval endpoint allows you to evaluate the quality of ranked search results.
+
+## Path and HTTP methods
+
+```
+GET <index_name>/_rank_eval 
+POST <index_name>/_rank_eval
+```
+
+## Query parameters
+
+Query parameters are optional.
+
+Parameter | Data Type | Description
+:--- | :---  | :---
+ignore_unavailable | Boolean | Defaults to `false`. When set to `false` the response body will return an error if an index is closed or missing.
+allow_no_indices | Boolean | Defaults to `true`. When set to `false` the response body will return an error if a wildcard expression points to indexes that are closed or missing.
+expand_wildcards | String | Expand wildcard expressions for indexes that are `open`, `closed`, `hidden`, `none`, or `all`.
+search_type | String | Set search type to either `query_then_fetch` or `dfs_query_then_fetch`.
+
+## Request fields
+
+The request body must contain at least one parameter.
+
+Field Type | Description
+:--- | :---  
+id | Document or template ID.
+requests | Set multiple search requests within the request field section.
+ratings | Document relevance score.
+k | The number of documents returned per query. Default is set to 10.
+relevant_rating_threshold | The threshold at which documents are considered relevant. Default is set to 1.
+normalize | Discounted cumulative gain will be calculated when set to `true`.
+maximum_relevance | Sets the maximum relevance score when using the expected reciprocal rank metric.
+ignore_unlabeled | Defaults to `false`. Unlabeled documents are ignored when set to `true`. 
+template_id | Template ID.
+params | Parameters used in the template.
+
+#### Sample request
+
+````json
+GET shakespeare/_rank_eval
+{
+  "requests": [
+    {
+      "id": "books_query",                        
+      "request": {                                              
+          "query": { "match": { "text": "thou" } }
+      },
+      "ratings": [                                              
+        { "_index": "shakespeare", "_id": "80", "rating": 0 },
+        { "_index": "shakespeare", "_id": "115", "rating": 1 },
+        { "_index": "shakespeare", "_id": "117", "rating": 2 }
+      ]
+    },
+    {
+      "id": "words_query",
+      "request": {
+        "query": { "match": { "text": "art" } }
+      },
+      "ratings": [
+        { "_index": "shakespeare", "_id": "115", "rating": 2 }
+      ]
+    }
+  ]
+}
+````
+
+#### Sample response
+
+````json
+{
+  "rank_eval": {
+    "metric_score": 0.7,
+      "details": {
+      "query_1": {                           
+        "metric_score": 0.9,                      
+        "unrated_docs": [                         
+          {
+            "_index": "shakespeare",
+            "_id": "1234567"
+          }, ...
+        ],
+        "hits": [
+          {
+            "hit": {                              
+              "_index": "shakespeare",
+              "_type": "page",
+              "_id": "1234567",
+              "_score": 5.123456789
+            },
+            "rating": 1
+          }, ...
+        ],
+        "metric_details": {                       
+          "precision": {
+            "relevant_docs_retrieved": 3,
+            "docs_retrieved": 6
+          }
+        }
+      },
+      "query_2": { [... ] }
+    },
+    "failures": { [... ] }
+  }
+}
+````