Add llms.txt to docs site for LLM crawler discoverability

[supriyar]

Stack from ghstack (oldest at bottom):

• Expand CODEOWNERS and add GitHub issue templates #4197
• -> Add llms.txt to docs site for LLM crawler discoverability #4196
• Add CLAUDE.md with AI agent instructions and quick reference #4195

Adds llms.txt following the llmstxt.org standard, served at
docs.pytorch.org/ao/llms.txt via html_extra_path in Sphinx config.
Helps Perplexity, ChatGPT, and other web-based LLM search engines
give accurate answers about torchao.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/4196

• 📄 Preview Python docs built from this PR

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

❌ 1 New Failure, 7 Unrelated Failures

As of commit cca7f26 with merge base 6f56403 ():

NEW FAILURE - The following job has failed:

• Run Regression Tests / test (CUDA 2.9, linux.g5.12xlarge.nvidia.gpu, torch==2.9.1, cuda, 12.6) / linux-job (gh)
  RuntimeError: Command docker exec -t 1215c2fb06ab01d2241107904c809c4e242435d179be008a4c9042f5e7c1ce4a /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Run Regression Tests / test (CUDA 2.10, linux.g5.12xlarge.nvidia.gpu, torch==2.10.0, cuda, 12.6) / linux-job (gh) (similar failure)
  test/quantization/pt2e/test_x86inductor_quantizer.py::TestQuantizePT2EX86Inductor::test_set_module_name_with_mixed_configs

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

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

• Run Regression Tests / test (CPU 2.10, linux.4xlarge, torch==2.10.0 --index-url https://download.pytorch.org/whl/cpu, cpu) / linux-job (gh) (trunk failure)
  test/quantization/pt2e/test_x86inductor_quantizer.py::TestQuantizePT2EX86Inductor::test_set_module_name_with_mixed_configs
• Run Regression Tests / test (CPU 2.8, linux.4xlarge, torch==2.8.0 --index-url https://download.pytorch.org/whl/cpu, cpu) / linux-job (gh) (trunk failure)
  test/quantization/pt2e/test_x86inductor_quantizer.py::TestQuantizePT2EX86Inductor::test_set_module_name_with_mixed_configs
• Run Regression Tests / test (CPU 2.9, linux.4xlarge, torch==2.9.1 --index-url https://download.pytorch.org/whl/cpu, cpu) / linux-job (gh) (trunk failure)
  test/quantization/pt2e/test_x86inductor_quantizer.py::TestQuantizePT2EX86Inductor::test_set_module_name_with_mixed_configs
• Run Regression Tests / test (CUDA 2.8, linux.g5.12xlarge.nvidia.gpu, torch==2.8.0, cuda, 12.6) / linux-job (gh) (trunk failure)
  test/quantization/pt2e/test_x86inductor_quantizer.py::TestQuantizePT2EX86Inductor::test_set_module_name_with_mixed_configs
• Run Regression Tests / test-nightly (CPU Nightly, linux.4xlarge, --pre torch --index-url https://download.pytorch.org/wh... / linux-job (gh) (trunk failure)
  test/quantization/pt2e/test_x86inductor_quantizer.py::TestQuantizePT2EX86Inductor::test_set_module_name_with_mixed_configs
• Run Regression Tests / test-nightly (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch --index-url https://downloa... / linux-job (gh) (trunk failure)
  test/quantization/pt2e/test_x86inductor_quantizer.py::TestQuantizePT2EX86Inductor::test_set_module_name_with_mixed_configs

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