🚀 Feature

Problems to solve:

• The markdown config (remark/rehype plugins) is often duplicated between multiple plugins
• There's no central place to configure markdown/MDX for the whole site at once
• It's not possible to build markdown-enhancement Docusaurus plugins
• It's not possible to disable the default emoji plugin (How to disable :emoji: replace? #5450)

We should allow users to configure default markdown config globally, change Docusaurus Markdown defaults, and make it possible to create markdown-related Docusaurus plugins.

A good example would be to create a "docusaurus-plugin-latex" so that scientists can more easily adopt Docusaurus without having to go through more complicated steps (see https://docs.theochu.com/docusaurus/latex/)

Related to #3018