[DEV DOCS] Documentation of Scheduler and Worker state machine

[fjetter]

Parent issue: dask/community#190

The scheduler state machine is documented in https://distributed.dask.org/en/stable/scheduling-state.html

This docs page has a few problems we should address. Below a few

• Reading flow is interrupted by object references which breaks the reading flow
• It is not discussed how the scheduler state machine maps onto the worker state machine although there is typically a 1:N mapping
• The page is placed in the "build understanding" section. We want to have a documentation which offers enough information for developers. We might want to consider splitting this off into user and developer sections
• The entire scheduler class reference is part of this page which is likely an error
• Many things changed about the implementation over the past months and we should ensure it is still up to date. In particular the list of stimuli feels out of date