Fix broken {include} paths rendering 3 empty doc pages

[AswaniSahoo]

Summary

Fix broken {include} directives in three documentation pages that currently
render as completely empty pages (<no title>) on the live docs site.

When backend docs were reorganized from flat files (e.g. backends-coreml.md)
into subdirectories (e.g. backends/coreml/coreml-overview.md), these three
platform-specific redirect pages were not updated to match the new paths:

• desktop-coreml.md — included non-existent backends-coreml.md
• desktop-mps.md — included non-existent backends-mps.md
• embedded-arm-ethos-u.md — included non-existent backends-arm-ethos-u.md

Updated include paths to match the correct subdirectory locations, consistent
with the working equivalents (ios-coreml.md, ios-mps.md,
embedded-arm-cortex-m.md).

Fixes #19445

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell @rascani @GregoryComer @AlannaBurke @mergennachin

Test plan

• Verified the three target files exist at the corrected paths
  (backends/coreml/coreml-overview.md, backends/mps/mps-overview.md,
  backends/arm-ethos-u/arm-ethos-u-overview.md)
• Confirmed the equivalent iOS/embedded pages (ios-coreml.md,
  ios-mps.md, embedded-arm-cortex-m.md) use the same corrected paths
  and render correctly on the live site
• No changes to source code; documentation-only PR

[pytorch-bot]

🔗 Helpful Links

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⏳ 6 Pending, 1 Unrelated Failure

As of commit 395d17c with merge base a92200f ():

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

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

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[Copilot]

Pull request overview

This PR fixes three documentation “redirect” pages that render empty on the live site due to broken MyST {include} paths after the backend docs were reorganized into subdirectories. It updates the includes to point at the current backend overview markdown files under docs/source/backends/..., matching the already-working iOS/embedded equivalents.

Changes:

• Update desktop-coreml.md to include backends/coreml/coreml-overview.md.
• Update desktop-mps.md to include backends/mps/mps-overview.md.
• Update embedded-arm-ethos-u.md to include backends/arm-ethos-u/arm-ethos-u-overview.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
docs/source/desktop-coreml.md	Fix {include} target to CoreML backend overview in the new backends/coreml/ location.
docs/source/desktop-mps.md	Fix {include} target to MPS backend overview in the new backends/mps/ location.
docs/source/embedded-arm-ethos-u.md	Fix {include} target to Arm Ethos-U backend overview in the new backends/arm-ethos-u/ location.

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[AswaniSahoo]

@pytorchbot label "release notes: none"