Add doxygen group for free function

Author: Mizux

Foo/include/foo/Foo.hpp

@@ -70,12 +70,15 @@ int pairJaggedArrayInput(std::vector<std::vector<std::pair<int, int>>> data);
 int pairJaggedArrayRefInput(const std::vector<std::vector<std::pair<int, int>>>& data);
 //! @}
 
+//! @defgroup FreeFunction Free function usage.
+//! @{
 /*! @brief Free function in foo namespace.
  * @param level Scope level.*/
 void freeFunction(int level);
 /*! @brief Free function in foo namespace.
  * @param level Scope level.*/
 void freeFunction(int64_t level);
+//! @}
 
 //! @brief Class Foo.
 class Foo {