Briefing for product to process trainee interviews

**Background**
Currently, our process of matching volunteer and trainee availabilities for interviews requires a lot of admin work, and thus we do not expect this process to be scalable once we get more trainees. Therefore, we need to think of a solution to help with this process.

**Acceptance criteria**
Write a briefing for a product that will assist in processing volunteer and trainee availabilities and help in booking the interviews with minimal admin work.

No source files, tests, or entry points are identified in the issue. Start with the Background and Acceptance criteria, then define the briefing's scope for processing volunteer and trainee availabilities and booking interviews; done is a completed product briefing.

索引モデルが issue の本文から書いたものです。