fix: default math font; ci; README

[RizhongLin]

Summary of this PR

Several users of our codebase have raised inquiries regarding math fonts. They have observed discrepancies between the math fonts utilized in this template and the purported "default" math fonts.

Consequently, I have made modifications to the following code:

\usepackage{ifthen}
\newboolean{useTimesNewRoman}
\setboolean{useTimesNewRoman}{false} % Set to false if you don't want to use Times New Roman 
                                     % or if you want to use the default math font (Latin Modern) for math symbols
\ifthenelse{\boolean{useTimesNewRoman}}{%
  \usepackage{amsmath,amsthm,amsfonts,amssymb,amscd}%
  \usepackage{fontspec}%
  \setmainfont{Times New Roman}%
}{%
  \usepackage{amsthm,amscd}%
  \usepackage{newtxtext,newtxmath}%
}

Hence, as indicated in the comments, if you desire to employ Times New Roman instead of Adobe New Roman, or if you wish to utilize the "default" math font, you can set the useTimesNewRoman variable to true.

If you have any suggestions or questions, please feel free to share them in the GitHub Discussions.

Should the successful integration of this PR lead to the closure of certain issues?

The specific issues discussed privately on QQ. In the future, it would be preferable to address such matters through GitHub Discussions. 👍🏻