The Substrate block authorship pipeline cannot reasonably be made into a Subsystem.

Related: #1143

In practice, we will have a subsystem that provides all information that we need to provide as InherentData to the block authorship system, so we just need an asynchronous interface for the block authorship system to fetch that data.

We should have a section in the guide on the actual block authorship pipeline and its interface to the overseer, and clarify that the block authorship subsystem is not actually responsible for authoring blocks.