Added docs for auth tokens in cookies (V17)

[kjac]

📋 Description

This is the V17 equivalent of #7604

📎 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)

V17.0

Deadline (if relevant)

27th of November if possible. That's the release date for V17.0.

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Looks great, @kjac 🙌 Will merge it on the 27th of November.