The ornamental design for a sideboard, as shown and described.
FIG. 1 is a top, left perspective view of a sideboard showing our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is an enlarged perspective detail of a portion of the sideboard of FIG. 1; and,
FIG. 5 is a top, right perspective view thereof, drawn to an enlarged scale with respect to FIGS. 1, 2, and 3.
The broken line depictions of structural elements in the design have been shown for the purpose of illustrating environment that forms no part of the claimed design.
The bottom, sides, and rear view, not shown, form no part of the claimed design.