docs: add picture of hosting architecture

[nhoening]

For the hosting section, we can show visually what nodes/VMs live in a typical FlexMeasures hosting:

• web workers
• queue workers
• cron job workers
• Postgres DB
• Redis DB

With some arrows in between, for read & write operations.

A possible load balancer node would help, as well, even if it is out of focus of this repo.