Improve the wiki multiprocess documentation

[ceyusa]

Right now the only reference to the multiprocess design is in https://github.com/servo/servo/wiki/Design#multi-process-architecture

It would be great if there are more design notes, or just maybe a diagram specifying the processes and their relations. Maybe aiming to code where currently it is done.

[asajeffrey]

Yeah, the docs for the process architecture keep drifting away from the impl. I wonder what we can do about this, the only way I can think of keeping them in sync is to move to having the docs in the source, but that's not ideal either.

[gterzian]

The specification of agent-cluster could anchor our architecture more firmly into a spec going forward.

Some overview is proposed at #24159 and I think its a good idea to update the Wiki as well.

We could also add some notes about the actual tools used, like https://github.com/servo/gaol and https://github.com/servo/ipc-channel/.