Write out stubs for most backing and availability subsystems (#1199)

* define subsystems skeleton

* Create a blank section for each subsystem in the initial batch

* write out basics for most subsystems

* Update roadmap/implementors-guide/guide.md

clean up language

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* ToC indentation

* clean up wording

* renaming block authorship (provisioning) to Provisioner

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>