Rewrite existing links to custom ops gdocs with the landing page

[zou3519]

Stack from ghstack (oldest at bottom):

• Beef up the allow_in_graph docs #127117
• Better graph break msg (and warning) on Dynamo x Python C++ extension #127301
• -> Rewrite existing links to custom ops gdocs with the landing page #127423
• New Custom Ops Documentation landing page #127400
• opcheck should be usable without optional dependencies #127292
• Add space in MetaFallbackKernel.cpp error message #127291

NB: these links will be live after the docs build happens, which is once
a day.

Test Plan:

• existing tests

cc @voznesenskym @penguinwu @EikanWang @jgong5 @Guobing-Chen @XiaobingSuper @zhuhaozhe @blzheng @wenzhe-nrv @jiayisunx @chenyang78 @kadeng @chauhang

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/127423

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• Upgrade MacOS runner to 14

✅ You can merge normally! (2 Unrelated Failures)

As of commit a8f54fd with merge base 0ff2f8b ():

UNSTABLE - The following jobs failed but were likely due to flakiness present on trunk and has been marked as unstable:

• inductor / linux-jammy-cpu-py3.8-gcc11-inductor / test (inductor_torchbench_cpu_smoketest_perf, 1, 1, linux.24xl.spr-metal, unstable) (gh) (#126993)
  Process completed with exit code 1.
• pull / linux-focal-cuda12.4-py3.10-gcc9-sm86 / test (default, 2, 5, linux.g5.4xlarge.nvidia.gpu, unstable) (gh) ()
  inductor/test_efficient_conv_bn_eval.py::EfficientConvBNEvalCudaTests::test_basic_cuda

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