feat: docs for pluggable Markdown processors

[Princesseuh]

Description (required)

Docs for withastro/astro#16149

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Tracked Files

File	Note
en/guides/integrations-guide/mdx.mdx	Source changed, localizations will be marked as outdated.
en/guides/markdown-content.mdx	Source changed, localizations will be marked as outdated.
en/guides/migrate-to-astro/index.mdx	Source changed, localizations will be marked as outdated.
en/guides/upgrade-to/v7.mdx	Localization changed, will be marked as complete.
en/recipes/external-links.mdx	Source changed, localizations will be marked as outdated.
en/recipes/modified-time.mdx	Source changed, localizations will be marked as outdated.
en/recipes/reading-time.mdx	Source changed, localizations will be marked as outdated.
en/reference/content-loader-reference.mdx	Source changed, localizations will be marked as outdated.
en/reference/modules/astro-content.mdx	Source changed, localizations will be marked as outdated.
en/reference/programmatic-reference.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	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.