Gets the heading's level: n for Hn; for H, the number of Sect elements it is nested in within its
Document or DocumentFragment, at least 1; 0 for a Title, the title of a document or high-level division,
above every numbered level.
public int Level { get; }
ISO 32000-2 Table 366 (Hn, H: "derived from the nesting of the logical structure", Title).