📝 add more API documentation

doc/mkdocs/docs/api/basic_json/unflatten.md

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+# basic_json::unflatten
+
+```cpp
+basic_json unflatten() const;
+```
+
+The function restores the arbitrary nesting of a JSON value that has been
+flattened before using the [`flatten()`](flatten.md) function. The JSON value must
+meet certain constraints:
+
+1. The value must be an object.
+2. The keys must be JSON pointers (see
+   [RFC 6901](https://tools.ietf.org/html/rfc6901))
+3. The mapped values must be primitive JSON types.
+    
+## Return value
+
+the original JSON from a flattened version
+
+## Exceptions
+
+The function can throw the following exceptions:
+
+- Throws [`type_error.314`](../../home/exceptions.md#jsonexceptiontype_error314) if value is not an object
+- Throws [`type_error.315`](../../home/exceptions.md#jsonexceptiontype_error315) if object values are not primitive
+
+## Exception safety
+
+Strong exception safety: if an exception occurs, the original value stays intact.
+
+## Complexity
+
+Linear in the size the JSON value.
+
+## Notes
+
+Empty objects and arrays are flattened by [`flatten()`](flatten.md) to `#!json null`
+values and can not unflattened to their original type. Apart from
+this example, for a JSON value `j`, the following is always true:
+`#!cpp j == j.flatten().unflatten()`.
+
+## Example
+
+??? example
+
+    The following code shows how a flattened JSON object is
+    unflattened into the original nested JSON object.
+    
+    ```cpp
+    --8<-- "examples/unflatten.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/unflatten.output"
+    ```
+
+## Version history
+
+- Added in version 2.0.0.