[PowerToysRun][Docs] Add SpeedTest to Third-Party plugins

[ruslanlap]

🚀 Add SpeedTest Plugin for PowerToys Run

This PR introduces the SpeedTest plugin—a new PowerToys Run extension that lets you run internet speed tests instantly, without opening a browser or separate application.

🔗 Learn more

Plugin	Author	Description
SpeedTest	ruslanlap	One-command internet speed tests with real-time results, modern UI, and shareable links.

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✨ Plugin Highlights

• One-command tests
  Run a full speed test (download, upload, ping, server) via PowerToys Run (). and hit Enter! (or change to or any other keyword in the settings)
• Modern, theme-aware UI
  WPF-based window adapts automatically to light or dark Windows modes.
• Live progress & shareable links
  Watch your test run in real time and copy a result URL when it finishes.
• All-local, privacy-first
  No third-party tracking—uses the bundled Speedtest CLI for 100% local execution.
• Robust error handling
  Friendly messages for network failures, permission issues, or unexpected errors.
• x64 & ARM64 builds
  Native-optimized releases for both CPU architectures.
• Open source & community-driven
  Contributions, issues, and feature requests are welcome—let's make it even better!

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Thank you for reviewing the SpeedTest plugin! 🚀
I'm excited to help PowerToys Run users check their internet speeds faster than ever.

[ruslanlap]

Hi everyone! 😊
Just wanted to kindly check in — this PR has been open for quite a few days now, and I’d be super grateful if someone could take a look when they have a moment. I’m really excited to contribute, and I hope everything looks good!
Thanks so much in advance! 🙏

Best regards,
ruslanlap