Enhance installation instructions for PowerToys

[JohnChiang2026]

Added details about installation paths for different PowerToys installers.

Summary of the Pull Request

Enhance installation instructions for PowerToys

PR Checklist

All things are able to merge. The README.md file is modified.

• Closes: #xxx

• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end-user-facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

I add some definitions about the differences between "Per User" installer and "Machine Wide" installer, which allows other people could easily indentify them. All changes could be able to merge according to the GitHub.

Validation Steps Performed

The platform operated automatically and all things were passed.

[Jay-o-Way]

Although "C:\Program Files" is common, this should be an EnvVar

[niels9001]

It feels like these instructions are pretty detailed, and would fit better in the Installation docs on Learn: https://learn.microsoft.com/en-us/windows/powertoys/install

Would you mind opening a PR there and add the changes?

Repo: https://github.com/MicrosoftDocs/windows-dev-docs/blob/docs/hub/powertoys/install.md