[Doc] Add Stack transform link in docs

[kurtamohler]

Stack wasn't getting included in the docs build because there was no link to it

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/2689

• 📄 Preview Python docs built from this PR

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

❌ 9 New Failures, 1 Unrelated Failure

As of commit 97305c5 with merge base ed656a1 ():

NEW FAILURES - The following jobs have failed:

• Build Windows Wheels / pytorch/rl / upload / wheel-py3_9-cpu (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_rl__3.9_cpu_
• Build Windows Wheels / pytorch/rl / upload / wheel-py3_9-cuda11_8 (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_rl__3.9_cu118_
• Build Windows Wheels / pytorch/rl / upload / wheel-py3_9-cuda12_4 (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_rl__3.9_cu124_
• Build Windows Wheels / pytorch/rl / upload / wheel-py3_9-cuda12_6 (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_rl__3.9_cu126_
• Continuous Benchmark (PR) / CPU Pytest benchmark (gh)
  Workflow failed! Resource not accessible by integration
• Continuous Benchmark (PR) / GPU Pytest benchmark (gh)
  Workflow failed! Resource not accessible by integration
• Habitat Tests on Linux / tests (3.9, 12.1) / linux-job (gh)
  AttributeError: _ARRAY_API not found
• Unit-tests on Linux / tests-gpu (3.11, 12.1) / linux-job (gh)
  test/test_loggers.py::TestMLFlowLogger::test_log_video[steps1]
• Unit-tests on Linux / tests-olddeps (3.8, 11.6) / linux-job (gh)
  test/test_storage_map.py::TestMCTSForest::test_forest_parent[simple-False]

BROKEN TRUNK - The following job failed but were present on the merge base:

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

• Unit-tests on Windows / unittests-cpu / windows-job (gh) (trunk failure)

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