docs(v2): add documentation about importing markdown

[Josh-Cena]

Motivation

The website has a hidden Markdown page example. It includes an example of importing markdown, which is not documented anywhere else on the website.

On Discord people often ask about reusing Markdown files, and I have to give them links to this hidden file. It would be great if it can be documented in the actual documentation.

I'm not sure if I'm putting this section in the right place...

Have you read the Contributing Guidelines on pull requests?

Yep

[netlify]

✔️ [V2]
Built without sensitive environment variables

🔨 Explore the source changes: d4d95fb

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😎 Browse the preview: https://deploy-preview-4958--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	60
🟢 Accessibility	97
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4958--docusaurus-2.netlify.app/

[slorber]

Thanks 👍 just added a warning for the TOC problem

[Josh-Cena]

Thanks 👍 just added a warning for the TOC problem

Yep, I realized that as well when I was testing this feature... I'm still stuck with understanding how the MDX loader works, so can't help rn with this

[slorber]

Thanks for prettier 😅

MDX basically converts md to a React component. In the end, Babel/Webpack just see a regular React comp.

The playground might help to understand: https://mdxjs.com/playground/