Documented UnattendedTelemetryLevel setting available for unattended installs

[AndyButland]

📋 Description

Adds details of a new UnattendedTelemetryLevel setting available for unattended installs, along with the --telemetry-level setting added to the dotnet new Umbraco template.

📎 Related Issues (if applicable)

umbraco/Umbraco-CMS#20249

✅ 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)

CMS 17.

Deadline (if relevant)

Should be published along with the Umbraco 17 beta due on October 15th.

[github-actions]

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