Update QAT README and API docstrings

[andrewor14]

Previously they pointed to the 0.7.0 code. Now they point to the corresponding API page on our docs. Also move the docstring from the private function to the public IntXQuantizationAwareTrainingConfig so it shows up on the doc page.

[pytorch-bot]

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❌ 5 New Failures

As of commit d46ad0c with merge base b1163dc ():

NEW FAILURES - The following jobs have failed:

• Run Regression Tests / test (CUDA 2.5.1, linux.g5.12xlarge.nvidia.gpu, torch==2.5.1 --index-url https://download.pytorch... / linux-job (gh)
  test/quantization/test_quant_api.py::TestQuantFlow::test_int8_wo_quant_save_load
• Run Regression Tests / test (CUDA 2.6, linux.g5.12xlarge.nvidia.gpu, torch==2.6.0, cuda, 12.6) / linux-job (gh)
  test/quantization/test_quant_api.py::TestQuantFlow::test_int8_wo_quant_save_load
• Run Regression Tests / test (CUDA 2.7, linux.g5.12xlarge.nvidia.gpu, torch==2.7.0, cuda, 12.6) / linux-job (gh)
  test/quantization/test_quant_api.py::TestQuantFlow::test_int8_wo_quant_save_load
• Run Regression Tests / test-nightly (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch --index-url https://downloa... / linux-job (gh)
  test/quantization/test_quant_api.py::TestQuantFlow::test_int8_wo_quant_save_load
• Run TorchAO Experimental Tests / test-mps-ops (macos-m1-stable) (gh)
  Process completed with exit code 127.

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