Docs: replace stale AppVeyor build references with branch-specific GitHub Actions artifact links

[Copilot]

The README’s “Current development builds” guidance pointed users to deprecated AppVeyor paths and did not clearly distinguish where to get artifacts for master vs development. This updates build documentation to the current GitHub Actions workflow views that expose downloadable artifacts for each branch context.

• What changed

  • Removed outdated AppVeyor references from the README header/status area.
  • Updated build badge/linking to GitHub Actions for master.
  • Rewrote Current development builds to provide explicit links for:
    • latest successful master push builds
    • latest successful development PR-targeted builds

• Resulting docs behavior

  • Users land on filtered workflow-run lists that map to the correct branch flow and can open runs to download artifacts from the Artifacts section.
  • Reduces ambiguity between stable (master) and in-flight (development) build outputs.

• Snippet (updated README section)

  ## Current development builds
  
  Download the latest successful GitHub Actions build, then open the run and download the artifacts from the **Artifacts** section:
  
  * **Master branch builds**: https://github.com/cmderdev/cmder/actions/workflows/build.yml?query=event%3Apush+branch%3Amaster+is%3Asuccess
  * **Development branch builds**: https://github.com/cmderdev/cmder/actions/workflows/build.yml?query=event%3Apull_request+branch%3Adevelopment+is%3Asuccess

[Copilot]

Pull request overview

Updates the README build documentation to remove deprecated AppVeyor references and direct users to the correct GitHub Actions workflow run lists for downloading build artifacts, distinguishing master push builds from development PR-targeted builds.

Changes:

• Removed AppVeyor build status badge/reference from the README header.
• Updated the build badge/link to point to GitHub Actions master push runs.
• Rewrote “Current development builds” to include explicit GitHub Actions links for successful master and development build artifacts.

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