aux/math: Group the files and structs in documentation

src/xrt/auxiliary/math/m_api.h

src/xrt/auxiliary/math/m_base.cpp

src/xrt/auxiliary/math/m_eigen_interop.h

src/xrt/auxiliary/math/m_optics.c

src/xrt/auxiliary/math/m_quatexpmap.cpp

src/xrt/auxiliary/util/u_documentation.h