Added docs for auth tokens in cookies (V16)

[kjac]

📋 Description

We are adding a new security feature to V16.4, allowing for strengthening the security for back-office users (specifically, access and refresh tokens for user sessions).

See umbraco/Umbraco-CMS#20779

This is the documentation for V16.4 - another PR will be created for V17.0, as this feature becomes mandatory in V17.

📎 Related Issues (if applicable)

None.

✅ Contributor Checklist

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

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

V16.4

Deadline (if relevant)

26th of November if possible. That's the release date for V16.4.

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Thanks for the PR, @kjac 🙌 The PR looks great.

I added a few suggestions to remove "we" and add an Oxford comma in the list of words.

Will merge it on the 26th of November.