It would really be best if you contacted a wildlife rehab group in your area ASAP. It can be dangerous to even give water to wildlife, and many animals have died by being fed by people who found them. All the wildlife groups I know of stress that an orphaned or injured animal should be brought to them immediately, and not fed anything. They have the know how and supplies to feed and care for any animals in need. Besides the fact that is is usually illegal for people other than wildlife rehabbers to keep any wild animals.
It is also possible that this rabbit is old enough to be on its own, and does not need any intervention. They leave the nest around 4 weeks of age. The fact that you saw it on you sidewalk might mean that it has ventured from the nest and can take care of itself. If that is the case, the only thing you can do is to keep any dogs or cats out of the area, to give it more of a chance to survive. You could look for a nest and see if there is one with other babies and return it, but that sounds doubtful. Calling a wildlife rehabber would be good, to get some advice, and see if they think it needs help.