Add Html Represenation to Boards

[gawebb-dstl]

Add _repr_html_ function to SvgWrapper. This allows chess boards to viewed in sphinx galleries.

def _repr_html_(self):
    return self`

[niklasf]

Sure, done. If this can be done in general, perhaps it's a good idea for Sphinx Galleries to also just support _repr_svg_?

[gawebb-dstl]

Thanks, I agree. I think there is a PR in Sphinx Galleries for that change but it has stalled.