Add "Author should include JSDoc" to PR template

[yusukebe]

What is the feature you are proposing?

Context: https://github.com/honojs/hono/pull/2615#issuecomment-2114090960

I'm thinking we should improve the PR template to tell every PR that adding a new feature should add TSDoc/JSDoc (if it's necessary) @yusukebe This way we can slowly migrate codebase to use TSDoc/JSDoc