Reorganize directories (#3462)

* 🚚 move files
* 🚚 rename doc folder to docs
* 🚚 rename test folder to tests

docs/mkdocs/docs/api/basic_json/is_structured.md

@@ -0,0 +1,63 @@
+# <small>nlohmann::basic_json::</small>is_structured
+
+```cpp
+constexpr bool is_structured() const noexcept;
+```
+
+This function returns `#!cpp true` if and only if the JSON type is structured (array or object).
+    
+## Return value
+
+`#!cpp true` if type is structured (array or object), `#!cpp false` otherwise.
+
+## Exception safety
+
+No-throw guarantee: this member function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Possible implementation
+
+```cpp
+constexpr bool is_primitive() const noexcept
+{
+    return is_array() || is_object();
+}
+```
+
+## Notes
+
+The term *structured* stems from [RFC 8259](https://tools.ietf.org/html/rfc8259):
+
+> JSON can represent four primitive types (strings, numbers, booleans, and null) and two structured types (objects and
+> arrays).
+
+Note that though strings are containers in C++, they are treated as primitive values in JSON.
+
+## Examples
+
+??? example
+
+    The following code exemplifies `is_structured()` for all JSON types.
+    
+    ```cpp
+    --8<-- "examples/is_structured.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/is_structured.output"
+    ```
+
+## See also
+
+- [is_primitive()](is_primitive.md) returns whether JSON value is primitive
+- [is_array()](is_array.md) returns whether value is an array
+- [is_object()](is_object.md) returns whether value is an object
+
+## Version history
+
+- Added in version 1.0.0.