docs: introduce KaTeX local copy plugin & Temml for math

[tats-u]

Pre-flight checklist

• [x] I have read the Contributing Guidelines on pull requests.
• [x] ~~If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.~~ N/A
• [x] ~~If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.~~ N/A

Motivation

It is a pain in the neck to manage and copy all .woff2 files of KaTeX in your repository to host them in your server. My plugin https://npmjs.com/package/@tats-u/docusaurus-plugin-copy-katex-assets will be helpful and worth introduced.

Ideally, this kind of plugin should be managed by Docusaurus itself, not an outsider like me.

Also, Temml is better than KaTeX if you prefer extremely small download size or non Latin Modern Math fonts and also worth introduced.

https://www.npmjs.com/package/@daiji256/rehype-mathml can be used instead of rehype-katex to use Temml.

Test Plan

Test links

Deploy preview: https://deploy-preview-11115--docusaurus-2.netlify.app/docs/markdown-features/math-equations/#mathml

Backports:

https://deploy-preview-11115--docusaurus-2.netlify.app/docs/3.7.0/markdown-features/math-equations/#mathml https://deploy-preview-11115--docusaurus-2.netlify.app/docs/3.6.3/markdown-features/math-equations/#mathml

Related issues/PRs

https://github.com/facebook/docusaurus/discussions/11100

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