arrow

GH-45485: [Dev] Simplify pull request template (#45599)

### Rationale for this change

It seems that the current comment based pull request template isn't read carefully.

### What changes are included in this PR?

* Remove explanations as comments
* Keep a basic introduction in the top as a normal text not a comment
* Use normal texts not comments for breaking changes and critical fix

### Are these changes tested?

No.

### Are there any user-facing changes?

No. This is only for developers.
* GitHub Issue: #45485

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>