Add warnings against exposing auth tokens in web apps

[kjac]

📋 Description

Following these recommendations from IETF (also mirrored by this article from Microsoft), it seems prudent to add explicit warnings to our documentation, discouraging site implementers from exposing auth tokens to web applications.

📎 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)

All active versions.

Deadline (if relevant)

Anytime 😄

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Excellent PR, @kjac 🙌 I added 1 suggestion to remove "please" and updated the link text to be more descriptive.