Fix broken {include} paths rendering 3 empty doc pages (#19585)

### 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

- [x] 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`)
- [x] 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
- [x] No changes to source code; documentation-only PR

Author: AswaniSahoo

docs/source/desktop-coreml.md

@@ -1 +1 @@
-```{include} backends-coreml.md
+```{include} backends/coreml/coreml-overview.md

docs/source/desktop-mps.md

@@ -1 +1 @@
-```{include} backends-mps.md
+```{include} backends/mps/mps-overview.md

docs/source/embedded-arm-ethos-u.md

@@ -1 +1 @@
-```{include} backends-arm-ethos-u.md
+```{include} backends/arm-ethos-u/arm-ethos-u-overview.md