Add smooth scrolling when clicking on section links

### Description

This is a very minor improvement that could be made to the website, I noticed that when you click on a particular section, it automatically just takes you to that section. Works as intended, but You can easily get lost where you are, since it magically teleports you there. Adding smooth scrolling would help with this issue, and let the reader know where the page is taking them.

### Motivation
Mild UI improvements for the user.

### Collaboration
I can build this out. Or if someone wants to do it

### Additional Context
Example of the thing I'm talking about. It just snaps to that section, adding smooth scrolling would scroll the page to that section, so the reader doesn't get lost.

issue にはファイルもテストも記載されていません。まず、ウェブサイトのセクションリンクの処理箇所を見つけ、現在のジャンプ動作を再現してください。完了の条件は、セクションリンクをクリックすると、目的の位置に瞬時に移動するのではなく、そこまで目に見えてスクロールすることです。

索引モデルが issue の本文から書いたものです。