Imply the existence of third party rehype plugins for unsupported math rendrers in README

[tats-u]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything or linked relevant results below
• I made sure the docs are up to date
• I included tests (or that’s not needed)

Description of changes

Related:

• Introduce rehype plugin support for Temml  #107
• Add rehype-mathml to list of plugins rehypejs/rehype#186
• Update explanation on @daiji256/rehype-mathml in plugins.md rehypejs/rehype#189

The current explanation may cause the reader to give up looking for third party plug-ins supporting math renderers other than MathJax or KaTeX.
There is @daiji256/rehype-mathml for Temml, which has been introduced in the plugin list in rehype, and we want to imply its existence (just implication because this README is mainly for official packages).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e99b9d0) to head (8fef61f).
Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #111   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        14    -1     
  Lines         1823      1831    +8     
=========================================
+ Hits          1823      1831    +8     

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[wooorm]

Thank you! :)