Add documentation for collapse, oversample, truncate_hits processors

Signed-off-by: Michael Froh <froh@amazon.com>

_search-plugins/search-pipelines/collapse-processor.md

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+---
+layout: default
+title: Collapse processor
+nav_order: 7
+has_children: false
+parent: Search processors
+grand_parent: Search pipelines
+---
+
+# Collapse processor
+
+The `collapse` response processor discards hits that have the same value for some field as a previous document in the result set.
+This is similar to the `collapse` parameter that can be passed in a search request, but the response processor is applied to the
+response after fetching from all shards. The `collapse` response processor may be used in conjunction with the `rescore` search
+request parameter or may be applied after a reranking response processor.
+
+Using the `collapse` response processor will likely result in fewer than `size` results being returned, since hits are discarded 
+from a set whose size is already less than or equal to `size`. To increase the likelihood of returning `size` hits, use the 
+`oversample` request processor and `truncate_hits` response processor, as shown in [this example]({{site.url}}{{site.baseurl}}/search-plugins/search-pipelines/truncate-hits-processor/#oversample-collapse-and-truncate-hits).
+
+## Request fields
+
+The following table lists all request fields.
+
+Field | Data type | Description
+:--- | :--- | :---
+`field` | String | The field whose value will be read from each returned search hit. Only the first hit for each given field value will be returned in the search response. Required.
+`context_prefix` | String | May be used to read the `original_size` variable from a specific scope to avoid collisions. Optional.
+`tag` | String | The processor's identifier. Optional.
+`description` | String | A description of the processor. Optional.
+`ignore_failure` | Boolean | If `true`, OpenSearch [ignores any failure]({{site.url}}{{site.baseurl}}/search-plugins/search-pipelines/creating-search-pipeline/#ignoring-processor-failures) of this processor and continues to run the remaining processors in the search pipeline. Optional. Default is `false`.
+
+## Example
+
+The following example demonstrates using a search pipeline with a `collapse` processor.
+
+### Setup
+
+Create many documents with a field that we'll use for collapsing:
+
+```json
+POST /_bulk
+{ "create":{"_index":"my_index","_id":1}}
+{ "title" : "document 1", "color":"blue" }
+{ "create":{"_index":"my_index","_id":2}}
+{ "title" : "document 2", "color":"blue" }
+{ "create":{"_index":"my_index","_id":3}}
+{ "title" : "document 3", "color":"red" }
+{ "create":{"_index":"my_index","_id":4}}
+{ "title" : "document 4", "color":"red" }
+{ "create":{"_index":"my_index","_id":5}}
+{ "title" : "document 5", "color":"yellow" }
+{ "create":{"_index":"my_index","_id":6}}
+{ "title" : "document 6", "color":"yellow" }
+{ "create":{"_index":"my_index","_id":7}}
+{ "title" : "document 7", "color":"orange" }
+{ "create":{"_index":"my_index","_id":8}}
+{ "title" : "document 8", "color":"orange" }
+{ "create":{"_index":"my_index","_id":9}}
+{ "title" : "document 9", "color":"green" }
+{ "create":{"_index":"my_index","_id":10}}
+{ "title" : "document 10", "color":"green" }
+``` 
+{% include copy-curl.html %}
+
+Create a pipeline that just collapses on the `color` field:
+
+```json
+PUT /_search/pipeline/collapse_pipeline
+{
+  "response_processors": [
+    {
+      "collapse" : {
+        "field": "color"
+      }
+    }
+  ]
+}
+```
+{% include copy-curl.html %}
+
+### Using a search pipeline
+
+In this example, we request the top 3 documents before collapsing on the "color" field. Since the first 2 documents have the same "color", the second one is discarded,
+and the request returns the first and third document:
+
+```json
+POST /my_index/_search?search_pipeline=collapse_pipeline
+{
+  "size": 3
+}
+```
+{% include copy-curl.html %}
+
+
+<details open markdown="block">
+  <summary>
+    Response
+  </summary>
+  {: .text-delta}
+```json
+{
+  "took" : 2,
+  "timed_out" : false,
+  "_shards" : {
+    "total" : 1,
+    "successful" : 1,
+    "skipped" : 0,
+    "failed" : 0
+  },
+  "hits" : {
+    "total" : {
+      "value" : 10,
+      "relation" : "eq"
+    },
+    "max_score" : 1.0,
+    "hits" : [
+      {
+        "_index" : "my_index",
+        "_id" : "1",
+        "_score" : 1.0,
+        "_source" : {
+          "title" : "document 1",
+          "color" : "blue"
+        }
+      },
+      {
+        "_index" : "my_index",
+        "_id" : "3",
+        "_score" : 1.0,
+        "_source" : {
+          "title" : "document 3",
+          "color" : "red"
+        }
+      }
+    ]
+  },
+  "profile" : {
+    "shards" : [ ]
+  }
+}
+```
+</details>