How do I repair a large area at side & bottom of external door
The only really practical solution here is to replace the door. If the dog has also chewed up the door framing in addition to the door then replacement of the door jamb and trimming is also in task list. 

After the replacements are in place make sure you address what has led the dog to do this. It is my experience that the animal is probably exhibiting this behavior because they are bored or have been confined to a small area behind the door where the chewing is taking place.