Fix Doxygen documentation

The doxygen documentation is broken:
https://sogno.energy/dpsim/doxygen/index.html

In local is working correctly with the submitted PR.

Related but not part of this problem: the actions worker is using -j$(nproc) as argument in the compilation... Should this be corrected in a new issue?

dpsim/.github/workflows/documentation.yaml

Lines 24 to 27 in d2cfcda

    
               - name: Make reference docs 
        
                 run: | 
        
                   make -j$(nproc) -C build docs 
        
                   make -j$(nproc) -C build docs_cxx

	- name: Make reference docs
	run: \|
	make -j$(nproc) -C build docs
	make -j$(nproc) -C build docs_cxx