Document debug_[request|reply] and debug_event

[SylvainCorlay]

This is adding documentation for

• debug_[request/reply], which are new messages on the control channel,
• debug_event, which are sent back debugging kernels to the front-end on IOPub.

[SylvainCorlay]

In this PR, we don't specify yet what debug_[request|reply] and debug_event messages contain, except that the former are on the Control channel (to prevent being queued after execution requests) and the latter on IOPub.

The actual protocol is almost certainly always going to be the Debug Adapter Protocol, and this is what we have implemented in the xeus-python kernel and in the (in-progress) work with in JupyterLab extensions.

All of it is functional with the latest versions of jupyter_client (5.3.1), jupyter_server (0.1.0) notebook (6.0), xeus-python (0.4.0). However, we need a (backward-compatible) PR merged in JupyterLab for the front-end extension to work.

I would argue that we should be able to move forward with merging this PR and then the JupyterLab PR. We already have a working implementation that demonstrates that this change can accomodate the Debugger Adapter Protocol. However, this is still not electing the Debugger Adapter Protocol per se at this point.

[MSeal]

LGTM