How can I recreate the "Sections" root that I deleted by mistake?
First, make sure it's really deleted and not just in the Trash:
If it's really deleted, then you can use Nuxeo Shell to recreate it (note that before Nuxeo 5.4.2, the Nuxeo Shell was only available as a separate download, since Nuxeo 5.4.2 it's available directly in the Admin Center).
See https://doc.nuxeo.com/x/E4dH for more about Nuxeo Shell, and https://doc.nuxeo.com/x/JIdH for the Automation commands (here we used Document.Create).