Best practice on where to define doxygen c++ documentations mainpage [duplicate]

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• How to make an introduction page with Doxygen 6 answers

When having a C++ project that only defines some classes and namespaces. Where should I put the mainpage documentation according to best practices for writing doxygen documentation? Placing it inside one of the header files in which the classes reside feels strange. Should I create a dedicated file that only contains this documentation?