Don't use `system-ui`, strip it from `--ifm-font-family-base`

### Have you read the Contributing Guidelines on issues?

- [x] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#issues).

### Motivation

`system-ui` is mainly designed for well i18ned web-based desktop apps (e.g. Electron-based). It is not designed for the font for documentation sites. e.g.:

1. `system-ui` in a Chinese page lοοκѕ lικе τһιѕ in Japanese Windows devices and vice versa. The shape of some characters are not intended.
2.

https://docusaurus.io/zh-CN/

Windows + Japanese locale

❌️ (system-ui)

The 1st character does not have an ideal shape as a standard Chinese printing font.
The 2nd character is incorrect. It's Japanese unofficial abbreviation form.

✅️ (modify CSS via F12 to remove system-ui)

(modify CSS via F12 to use the font used as system-ui in Chinese Windows)

https://docusaurus-i18n-staging.netlify.app/ja/docs/migration/v3

❌️ (system-ui in Japanese Windows)

Too narrow. This font is helpful only in UI elements whose widths are limited.

This font is designed to prevent the width of UI elements intended for English text from expanding excessively when Japanese text is inserted. It is not suitable for normal use outside of PowerPoint.

❌️💀❌️ (replace system-ui with a default UI font in Chinese Windows to simulate Chinese locale)

Looks very ugly as Japanese text. Some character have incorrect shape (e.g. 非, 直 → 非, 直 )

✅️ (remove system-ui)

The shape of Japanese (and Chinese) character for the main text should be square like this.

This should be fixed by v4 release.

https://infinnie.github.io/blog/2017/systemui.html
https://heistak.github.io/your-code-displays-japanese-wrong/
https://github.com/w3c/csswg-drafts/issues/3658
github.com/mdn/content/issues/41244
https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/font-family#system-ui

### Self-service

- [ ] I'd be willing to do some initial work on this proposal myself.