Added information about default groups when auto-linking backoffice accounts from external login providers

[AndyButland]

📋 Description

Added information about default groups when auto-linking backoffice accounts from external login providers.

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
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• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

CMS

Deadline (if relevant)

Any time