Arm backend: Fix stale docgen generation

[Christoffer-JL]

Update the Ethos-U docgen template to point to the correct tutorial path. This keeps pre-push from regenerating docs with the stale tutorial URL.

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

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19551

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❌ 1 New Failure, 3 Unrelated Failures

As of commit 3ec2792 with merge base 9ccbc4a ():

NEW FAILURE - The following job has failed:

• Lint / link-check / lint-xrefs (gh)

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)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe

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