docs: elaborate on the usage of MDX plugins

[cerkiewny]

Motivation

Fixes #5492

Have you read the Contributing Guidelines on pull requests?

Yes.

Test Plan

Document only change - manual tests. Preview link: https://deploy-preview-5766--docusaurus-2.netlify.app/docs/next/markdown-features/plugins/

[netlify]

✔️ [V2]
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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	67
🟢 Accessibility	98
🟢 Best practices	93
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5766--docusaurus-2.netlify.app/

[Josh-Cena]

Perhaps also linking to Docusaurus's own Remark plugins? https://github.com/facebook/docusaurus/tree/main/packages/docusaurus-mdx-loader/src/remark

Also mention in what cases may users want to create a Remark plugin (a common one: when they want to transform a node into something else other than the default one)

[slorber]

Some sentences feel a bit weird, duplicate, or too long/complex.

Please try to write something more concise, to the point, and consistent with the rest of the doc

[slorber]

Seems better but some things are wrong

Also I'm not a native English speaker but I think there are some grammatical issues

[Josh-Cena]

@cerkiewny Does the new writeup make sense to you? Is there any other reminder that you want to mention?

[cerkiewny]

@cerkiewny Does the new writeup make sense to you? Is there any other reminder that you want to mention?

Makes sense, I see you have added the section creating example plugin which for sure helps.

[slorber]

Thanks, that looks better and consistent 👍 we can always improve a bit more later