[docs] Publish images on docker hub again and clarify where images are published, clarify also what docker-openwisp is for

[nemesifier]

We should clarify where images are published (git container registry) in the README.

It may be good to also publish the images again on Docker Hub since it's the most common way to publish images and it gives us visibility.

We should also clarify the main purpose of docker-openwisp so that users can clearly understand that this project is focused on allowing OpenWISP to scale horizontally (explain that different components have different scaling requirements depending on the nature of the networking service implemented and for that reason there different images for each core component).