docs: add pointers to IPython and kernel options The docstrings of embed_kernel, start_ipython, start_kernel and embed mention that a traitlets Config can be passed as 'config' to configure the app / kernel, but the available options are not mentioned. Adjust write_doc in 'autogen_config.py' so that it adds a label before the title of each documented class, and use that label to link to the IPython terminal options from the docstrings of start_ipython and embed, and to the kernel options from the docstrings of start_kernel and embed_kernel.