Update doc/manifest/README.md to include 1.12 schema

[KarbitsCode]

Checklist for Pull Requests

• Have you signed the Contributor License Agreement?
• Is there a linked Issue? If so, fill in the Issue number below.
  • Resolves #[Issue Number]

Manifests

• Have you checked that there aren't other open pull requests for the same manifest update/change?
• This PR only modifies one (1) manifest
• Have you validated your manifest locally with winget validate --manifest <path>?
• Have you tested your manifest locally with winget install --manifest <path>?
• Does your manifest conform to the 1.12 schema?

Note: <path> is the directory's name containing the manifest you're submitting.

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Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Updates the manifest documentation index to reference the latest WinGet manifest schema documentation, aligning the doc entrypoint with the PR template’s recommended schema version.

Changes:

• Added a link to the 1.12.0 manifest schema documentation in doc/manifest/README.md.