Improve README about building the Docker image from a server ZIP built from sources

Currently, we have a dedicated README explaining how to build the Docker images from the nuxeo/docker directory, but it is unclear how to build the image from the nuxeo root directory by building a fresh server ZIP.

Let's update the root nuxeo README with a Docker section providing such useful commands, something like:

• Long version: builds everything, including packages (useful if there's a need to install some):

  mvn -nsu -Pdistrib,docker -DskipTests install
  

• Short version: build the nuxeo/nuxeo Docker image with its dependencies only: server modules, server ZIP, etc:

  mvn -nsu -Pdistrib,docker -pl docker/nuxeo -am -DskipTests install
  

Let's also do some cleanup in both:

• The nuxeo README: remove/rework some sections: Sub-Modules Organization, QA, Deploying, etc.
• The nuxeo/docker README, taking into account the removal of the nuxeo/slim image, see NXP-29684.