docs: add more pointers to configuration of embedded IPython The "Running IPython from Python" section of the docs under "Configuring IPython" gives a nice example of how to set IPython configuration options when embedding IPython. Add more pointer to this part of the documentation by linking to it from the docstrings of IPython.{embed_kernel, start_ipython, start_kernel} and IPython.terminal.embed.embed. While at it, consistenly refer to the config argument as "a traitlets :class:`Config` object". Finally, also mention that options can be set with a 'Config' object in the small paragraph under "IPython options".