You can put example source code in a special path defined in the doxygen config under EXAMPLE_PATH, and then insert examples with the @example tag.
Doxygen will then generate an extra page containing the source of the example. It will also set a link to it from the class documentation containing the example tag.
Alternatively if you want to use small code snippets you can insert them with @code ... @endcode
The documentation for this is here:
doxygen documentation?