IPC docs

[Sjors]

I have a local branch that adds IPC docs, similar to RPC.

I plan to PR it after #1245.

[stickies-v]

As far as I'm aware, we don't have any stable IPC interfaces yet, correct? I think it makes sense to document the IPC interface on the website regardless, as long as we make (the lack of) stability and versioning guarantees etc very obvious,