Fix memilio logo in doxygen documentation

[reneSchm]

Changes and Information

The github pages documentation (https://scicompmod.github.io/memilio/documentation/index.html) does not display the memilio logo correctly, since the image file is not available. Only its name is printed.

The same is true for manually generated doxygen files.

This can be fixed by setting the IMAGE_PATH variable in the docs/Doxyfile to ../docs/memilio-small.png

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