Switching-Doxygen-versions.md

+The documentation is built by running make doc in the dumux build folder. One way to switch doxygen versions is to:
+
+* download the doxygen git repository from `https://github.com/doxygen/doxygen.git`
+* checkout the branch you want, e.g. `Release_1_8_17`
+* the first time do: `mkdir build`, `cd build`, `cmake -G "Unix Makefiles"` ..
+* always do in the doxygen build directory: `make, sudo make install`
+* the first time do: `export PATH=/usr/local/bin/doxygen:$PATH`
+* delete the dumux build folder and rerun dunecontrol for dumux `./dune-common/bin/dunecontrol --opts=./dumux/cmake.opts --only=dumux all`
+
+After Changes in `scr/doxygen.cpp` delete the build folder an recreate it.
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