feat: use dd4hep_add_plugin and podio_add_datamodel_core_lib

This introduces some consistency in how the libraries are built, to avoid issues with missing linked libraries.

docs/CMakeLists.txt

-# from https://devblogs.microsoft.com/cppblog/clear-functional-c-documentation-with-sphinx-breathe-doxygen-cmake/ 
+# from https://devblogs.microsoft.com/cppblog/clear-functional-c-documentation-with-sphinx-breathe-doxygen-cmake/
 find_package(Doxygen REQUIRED)
 
 # Find all the public headers