docs/website/images/openhomefoundation-logo.png: crop for better layout

The Open Home Foundation logo is in a white area that is too large for
how we display it on the sponsors page. As a result, it ends up at the
bottom of the logo area and it looks very unbalanced.

Crop the logo to a tigher area. Since it is going to end up being
scaled, it's difficult to predict what the best size it is, but cropping
at a height of 300 pixels gives a visually pleasing layout at at least
some resolutions.

Signed-off-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Julien Olivain <ju.o@free.fr>