Command for creating Site skeletons container should take section into account.

Context: Upgrading from v3.0 to v3.1.

On version 3.1, when you install from scratch, the Site skeletons container is assigned to the Site skeleton section. When upgrading from version 3.0 we recommend to use the command php ./bin/console ezplatform:site-factory:create-site-skeletons-container which creates the container in Standard section (ref. https://doc.ezplatform.com/en/master/guide/site_factory/#configure-site-skeleton).

We have a few possibile solutions here:

1. The section has to be created beforehand and the command takes the section id as a parameter.
2. The command takes the section id (and name) and handles creating the section.
3. The section has to be created beforehand. There is no change to the command. We document that the integrator has to manually change the section of the Site skeletons container.

Rationale: We want to keep order under Content structure and in Search and keep the skeletons separated.