fix: Update landing page link down button position and smooth scrolling

[3DRX]

What type of PR is this?

fix: English page link down button easily overlaps with other buttons. feat: Add smooth scrolling for link down button.

Check the PR title.

• [x] This PR title match the format: \<type>(optional scope): \<description>
• [x] The description of this PR title is user-oriented and clear enough for others to understand.

More detailed description for this PR(en: English/zh: Chinese).

en:

Before: scroll down button will overlap with other buttons in regular sizes (90%-110%) on a regular screen (1920x1080) when browser full screened.

Screencast from 2024-07-18 23-41-45.webm

After: no overlap, and with smooth scrolling down animation.

Screencast from 2024-07-18 23-46-06.webm

cn: 此前 Landing Page 的向下翻页按钮可能会挡住 Get Started 和 Github 按钮使其无法被点击（在常用的浏览器窗口大小和缩放比例下：1920x1080, 90%-110%）。这个 pr 调整了 css 并添加了平滑的向下滚动动画。

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