Overhaul desktop/laptop platform docs: platform-specific guides for m…

[AlannaBurke]

Summary

Refocuses the Desktop & Laptop Platform section on native execution across
macOS, Linux, and Windows. Replaces thin {include} wrappers and a duplicate
landing page with clear, platform-specific guides grounded in the official
ExecuTorch source docs.

Changes

New Files (3 )

• docs/source/desktop-macos.md — macOS guide covering prerequisites
  (Xcode, macOS version requirements), Apple Silicon vs Intel considerations,
  all three backends (Core ML, MPS, XNNPACK), and build steps using the macos
  CMake preset.
• docs/source/desktop-linux.md — Linux guide covering prerequisites
  (supported distros, compiler requirements), XNNPACK and OpenVINO backends,
  and build steps using the linux CMake preset.
• docs/source/desktop-windows.md — Windows guide covering prerequisites
  (Visual Studio 2022 + Clang-CL), the critical symlinks setup step, XNNPACK
  and OpenVINO backends, and WSL as an alternative.

Modified Files (2)

• docs/source/desktop-section.md — Overhauled landing page with a 3-card
  OS selector grid, a backend-by-platform support table, and a clean toctree
  pointing to the three new platform guides.
• docs/source/edge-platforms-section.md — Updated Desktop section
  description to reflect the new platform-specific structure.

Deleted Files (6)

• desktop-backends.md — Replaced by the three platform-specific guides.
• desktop-coreml.md, desktop-mps.md, desktop-xnnpack.md, desktop-openvino.md —
  Thin {include} wrappers removed; content is now properly linked from the
  platform guides.
• platforms-desktop.md — Duplicate of the landing page; removed.

What's Sourced

All content is derived directly from the authoritative ExecuTorch source docs:

• System requirements and OS versions from using-executorch-building-from-source.md
• CoreML requirements from backends/coreml/coreml-overview.md
• MPS requirements from backends/mps/mps-overview.md
• XNNPACK target requirements from backends/xnnpack/xnnpack-overview.md
• OpenVINO setup from build-run-openvino.md
• CMake presets from using-executorch-building-from-source.md

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19101

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Rolling out OSDC (ARC) runners on pull & trunk workflows in PyTorch main

❌ 2 New Failures, 2 Unrelated Failures

As of commit f394e11 with merge base eef7921 ():

NEW FAILURES - The following jobs have failed:

• Build Presets / linux (llm, linux.2xlarge, executorch-ubuntu-22.04-clang12) / build (gh)
• Lint / link-check / lint-xrefs (gh)
  Process completed with exit code 1.

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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