feat: add information on new middleware modes

[leekeh]

Description (required)

Update docs based on changes in linked issue.

For Astro version: 6.x. See astro PR #15686.

[astrobot-houston]

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Tracked Files

File	Note
en/guides/authentication.mdx	Source changed, localizations will be marked as outdated.
en/guides/integrations-guide/node.mdx	Source changed, localizations will be marked as outdated.
en/guides/middleware.mdx	Source changed, localizations will be marked as outdated.
en/reference/adapter-reference.mdx	Source changed, localizations will be marked as outdated.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.

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