Add kernel and session architecture diagram

[jtpio]

There is currently a good overview of the different kernel related concepts in the services README.md

I wonder whether there could also be a diagram that explains how all the different components talk to each other. Maybe something similar to the one posted in #7252 (comment):

Maybe the exact same diagram could be used, unless some of the inner details have changed.

[vidartf]

I think this is still accurate in explaining how things are in the most common case in lab, but @jasongrout would have to confirm.

[jasongrout]

The "ClientSession" is now "SessionContext", but other than that, I think it is still essentially accurate. I suppose it's a little confusing that SessionConnection contains KernelConnection, but SessionContext is off to the side. In reality, SessionContext has a reference to a SessionConnection (or null), which in turn has a reference to a KernelConnection (or null). Thanks!

[jasongrout]

And I suppose all the "WS Bridge" boxes should be inside a "Jupyter Server" box to indicate that they are part of the server.