CMS: Revise icons documentation with new sources and example

[iOvergaard]

📋 Description

• Updated icon sources and added usage example.
• Replaced link to real documentation

📎 Related Issues (if applicable)

Partly fixes umbraco/Umbraco-CMS#19975

✅ 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)

16

Deadline (if relevant)

ASAP

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide