Japanese: Translate code comments on the getting started page to JP

[wongjas]

The code comments on this page are still in English: https://slack.dev/bolt-js/ja-jp/tutorial/getting-started.

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• example code related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.