GH-33923: [Docs] Tensor canonical extension type specification

docs/source/format/CanonicalExtensions.rst

@@ -72,4 +72,24 @@ same rules as laid out above, and provide backwards compatibility guarantees.
 Official List
 =============
 
-No canonical extension types have been standardized yet.
+Fixed size tensor
+=================
+
+* Extension name: `arrow.fixed_size_tensor`.
+
+* The storage type of the extension: ``FixedSizeList``.
+
+* Extension type parameters:
+
+  * **value_type** = Arrow DataType of the tensor elements
+  * **shape** = shape of the contained tensors as a tuple
+  * **is_row_major** = boolean indicating the order of elements in memory
+
+* Description of the serialization:
+
+  The metadata must be a valid JSON object including:
+
+  * shape of the contained tensors as an array with key “shape”,
+  * string defining the order of elements in memory with key “order”.
+
+  For example: `{ "shape": [2, 5], "is_row_major": True }`