Fix left nav TOC for backends overview page in main

[pytorchbot]

The backends-overview page was only showing 6 backends instead of 13. This regression was introduced in commit cdc2701 (NXP Backend: Update documentation to the new scheme #15219) which inadvertently reverted correct toctree paths to incorrect ones during a table reformatting.

• Toctree entries used old flat paths (e.g., backends-xnnpack) instead of the new subdirectory paths (e.g., backends/xnnpack/xnnpack-overview)
• A circular toctree dependency between nxp-overview.md and nxp-dim-order.md caused Sphinx build failures

Test Plan: Built sphinx locally and rendered page

https://docs-preview.pytorch.org/pytorch/executorch/16861/backends-section.html

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16869

• 📄 Preview Python docs built from this PR

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

❌ 8 New Failures

As of commit 343b0c1 with merge base 7492d0d ():

NEW FAILURES - The following jobs have failed:

• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t b1ecbe5a914797635c9d23a3308852e6bf1a3cca31bec3e0eeca5ae73e5122cc /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 2394baced07f7b5e5951418a3214f3fa71673bd8109a9909cdb8787876368d35 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 10a451396122bbb90facc891b90d78b75f110837dfcecb60a5571d6945296696 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t eb1dc6fca53e48255b254e1bb3752422d8e28f4379a0d84d949b457d9f66f0e5 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, quantized-int4-weight-only) / linux-job (gh)
  RuntimeError: Command docker exec -t e45a322aa7a6b45f204ff61e4898093a838f86e237d84a39815ce324cb1fb88b /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 74ff9b230bce3a9c4ed0f07daff1183aa6a251fb8de796f453777a7a1040095f /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t 92ad53585c35d742a3dd5961544276d06ef70d6bc7a2741702f6b88c8fccd522 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, quantized-int4-weight-only) / linux-job (gh)
  RuntimeError: Command docker exec -t 929320cd7c28293c6bd7b71cd2e0f2edba97b4d8facf1d2461d93ae5d44e999b /exec failed with exit code 1

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