Initial documentation of the Kernel protocol

[JohanMabille]

This is more or less the documentation that we currently have in jupyter_client.

I need to setup the readthedocs project so that it can be built. In the meantime, any feedback is welcome.

[Zsailer]

Thank you @JohanMabille! I think it's a good idea to separate the protocol from jupyter_client documentation.

That said, we should carefully consider how we can redirect folks from those old documentation pages to these new pages. The jupyter_client repo has been around awhile, and I'd bet there are many places on the internet where those old pages are referenced. We should do our best not to break those links...

[Zsailer]

That said, we should carefully consider how we can redirect folks from those old documentation pages to these new pages. The jupyter_client repo has been around awhile, and I'd bet there are many places on the internet where those old pages are referenced. We should do our best not to break those links...

I don't think this should block us from merging this PR. Let's keep this as a follow-up action item.