Amazon.com Inc. has battled Apple Inc. over digital books, digital music and mobile applications. Now the two companies are taking their clash to another front: the tablet market. Amazon plans to release a tablet computer by October, people familiar with the matter said, intensifying its rivalry with Apple’s iPad, reports the Wall Street Journal. While Amazon has long offered digital content on its website, it has lacked much of the hardware to go with it. Now the Seattle company hopes customers will use its tablet to buy and rent that content, said people familiar with its thinking…