Adds info about umbLink UFM component

[skttl]

📋 Description

I was just about to write my own UFM component for handling link pickers, when I noticed there already was one.

✅ 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)

16.0.0

[eshanrnh]

The PR looks great, @skttl 🙌