DOC: scheduler: add a diagram showing the different queues and their usages

The scheduler has become complex over time and the latest updates were a
good opportunity to document it. This diagram shows the time-based wait
queue(s), the priority-based run queue(s), and the class-based tasklet
queues, trying to emphasize what is local-only and what is shared between
threads. The diagram is provided in .fig, .svg, .png, and .pdf.
4 files changed