Fix mathjax overflow

[sli1989]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• Tests for the changes have been added (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs have been added / updated (for bug fixes / features).

PR Type

What kind of change does this PR introduce?

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

If a formula rendered by Mathjax is beyond the width of the screen, the default behavior will result in a horizontal scrollbar, which is very unattractive.

Issue Number(s): #460

What is the new behavior?

This pr display Mathjax within overflow.

• Screens with this changes: N/A
• Link to demo site with this changes: https://test.saili.science/posts/math-test/

How to use?

In NexT _config.yml:

...

Does this PR introduce a breaking change?

• Yes.
• No.