Add composite aggregation documentation

_aggregations/bucket/composite.md

@@ -0,0 +1,79 @@
+---
+layout: default
+title: Composite
+parent: Bucket aggregations
+grand_parent: Aggregations
+nav_order: 20
+has_children: true
+---
+
+# Composite
+
+The `composite` aggregation is a multi-bucket aggregation that creates composite buckets from different sources. It is useful for efficiently paginating multi-level aggregations and retrieving all buckets. Composite buckets are built from combinations of values extracted from documents for each specified source field.
+
+## Syntax
+
+```json
+{
+  "composite": {
+    "sources": [
+      {
+        "source_field_1": {
+          "terms": {
+            "field": "field_name"
+          }
+        }
+      },
+      {
+        "source_field_2": {
+          "terms": {
+            "field": "another_field_name"
+          }
+        }
+      }
+    ]
+  }
+}
+```
+{% include copy-curl.html %}
+
+Property | Description |
+---------|------------|
+`composite` | The aggregation type.
+`sources ` | An array of source objects, where each object defines a source field for the composite buckets.
+`terms` | The subaggregation type used to extract the values from the specified field for each source. 
+`field` | The field name in your documents from which the values will be extracted for the corresponding source.
+
+For example, consider the following document: 
+
+```json
+{
+  "product": "T-Shirt",
+  "category": "Clothing",
+  "brand": "Acme",
+  "price": 19.99,
+  "sizes": ["S", "M", "L"],
+  "colors": ["red", "blue"]
+}
+```
+{% include copy-curl.html %}
+
+Using `sizes` and `colors` as source fields for the aggregation results in the following composite buckets: 
+
+```json
+{ "sizes": "S", "colors": "red" }
+{ "sizes": "S", "colors": "blue" }
+{ "sizes": "M", "colors": "red" }
+{ "sizes": "M", "colors": "blue" }
+{ "sizes": "L", "colors": "red" }
+{ "sizes": "L", "colors": "blue" }
+```
+{% include copy-curl.html %}
+
+## Compatibility and limitations 
+
+<SME: What version of OpenSearch is this compatible with? What are the limitations?>
+
+## Performance considerations
+
+<What are the performance implications or best practices for using this aggregation?>