Only find doxygen if building the docs

[tmadlener]

Ran into the issue of trying to build without documentation but cmake still failed. The problem in my case is that doxygen is present, but dot is to be missing from the build environment. This makes find_package(Doxygen) fail because cmake treats it as a required component by default: https://cmake.org/cmake/help/v3.23/module/FindDoxygen.html