MoonMoon You can drag and drop the tags in the admin panel. When dropping a tag under another primary tag, you can insert it normally or shift it to the right (the drop zone will shift) using the mouse. The tags show up as a tree similar to how they appear in the sidebar on the homepage here.
It might be a bit difficult to achieve on mobile with touch controls.
EDIT: Unfortunately I don't have any screenshot ready. We should probably add a screenshot of this to the documentation.
