docs: add telemetry documentation to extensions page

[thomhurst]

Summary

• List all built-in extensions (Code Coverage, TRX Reports, Telemetry) at the top of the extensions page
• Add tip explaining how to opt out of built-in extensions by using TUnit.Engine and TUnit.Assertions packages separately
• Add Telemetry section with environment variable opt-out instructions
• Link to Microsoft's official telemetry documentation

Closes #4424

🤖 Generated with Claude Code

[thomhurst]

Summary

Adds telemetry documentation to the extensions page, explaining built-in extensions and how to opt out.

Critical Issues

None found ✅

Suggestions

Minor inconsistency on line 7:
The text "You don't need to install them separately!" is now slightly misleading since the new content explains how to opt out by NOT using the meta package. Consider updating line 7 to match the new structure:

-The following extensions are **automatically included** when you install the **TUnit** meta package. You don't need to install them separately!
+The following extensions are **automatically included** when you install the **TUnit** meta package:

This change is already in the PR diff, so no action needed! 🎉

Previous Review Status

No previous review comments found.

Verdict

✅ APPROVE - No critical issues

The PR accurately documents the telemetry extension and provides clear opt-out instructions. The Microsoft documentation link is valid and the environment variable names are correct. This properly addresses issue #4424.