code: func (f *Fpdf) Bookmark(txtStr string, level int, y float64) {
  	if y == -1 {
  		y = f.y
  	}
  	f.outlines = append(f.outlines, outlineType{text: txtStr, level: level, y: y, p: f.PageNo(), prev: -1, last: -1, next: -1, first: -1})
  }
docs: // Bookmark sets a bookmark that will be displayed in a sidebar outline. txtStr
  // is the title of the bookmark. level specifies the level of the bookmark in
  // the outline; 0 is the top level, 1 is just below, and so on. y specifies the
  // vertical position of the bookmark destination in the current page; -1
  // indicates the current position.