feat: document recma plugins new spot

[Princesseuh]

Description (required)

withastro/astro#17262 moves recma plugins into the remark processor directly, deprecating the previous MDX integration option. This is quite similar to remarkPlugins, rehypePlugins etc.

Anecdotally, I've never seen anyone use a recma plugin, ha

[astrobot-houston]

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en/guides/integrations-guide/mdx.mdx	Source changed, localizations will be marked as outdated.
en/guides/markdown-content.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[ArmandPhilippot]

I couldn't check the preview (see my suggestion for the fix), but this seems good to me! Thank you!

[github-actions]

Preview deployment

✅ Deployment complete!

• Latest commit: 693eeda
• Preview: https://docs-mdx-recma-plugins-processor.previews.docs.astro.build
• Workflow run: View logs

[ArmandPhilippot]

Preview also LGTM! 🙌🏽