Improve documentation for IPython.core.debugger and IPython.terminal.debugger

The documentation for `IPython.core.debugger` states that it is deprecated but it doesn't provide any information about what should be used instead. A few questions:
- Was `IPython.core.debugger` deprecated in favour of `IPython.terminal.debugger`?
- If so, what should be used in jupyter notebooks (`IPython.terminal.debugger.set_trace()` doesn't appear to work when used in a notebook)?

Read the existing documentation for IPython.core.debugger and IPython.terminal.debugger first, then check how IPython.terminal.debugger.set_trace() behaves in a Jupyter notebook. Update the documentation to identify the intended replacement for the deprecated module and explain the appropriate notebook usage.

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