the docs do not current reflect the new net-install and packages modules
changes well. This updates them to explain the new keys in the
configuration files.
Package groups are divided into critical and non-critical
depending on whether we want all Calamares to fail if installing
a package in the group fails, or we are okay with just logging a
warning.
The distinction is configured in the YAML file listing the package
groups. By default, all groups are critical, to keep supporting
the previous behaviour.
