A section title: a level and the text of the heading.
level is an integer from 1 to 6 - the range every document format that
has headings agrees on, and the range a renderer can map onto whatever it
builds. It is a number rather than a style name because what a heading
means is its depth in the document; how big it looks is the renderer's.
iex> node = %{type: "heading", key: "account_heading", level: 1, text: "Create your account"}
iex> Riddler.Screens.Type.Heading.validate(node)
[]