feat: replace fandol font with ubuntu in ctex

[xiaohanyu]

Description

Fandol font in ctex do not have very good support for rarely-seen character (生僻字)，such as 瑢, 樭, it will show an F inside a rectangle:

Example:

The root cause is, the fandol font set that we use right now do not seem support these rarely-seen characters. ctex packages provides other options for font size:

ubuntu and founder font seems to be free to commercial use, let's use ubuntu first. Later we may allow user to choose font based on their own needs.

Ref:

• Ubuntu font set in Chinese: https://www.fyears.org/2017/04/source-han-sans-and-source-han-serif.html (Chinese post)
• Fandol font also has some legal risk for commercial use

[Optional] Possible Solutions

NA

Acceptance Criteria

• rarely-seen characters in CJK should be handled correctly and show in PDF

Todo list

NA

[xiaohanyu]

Fixed, preview: