[CI] Add path-based triggers for niche workflows

[vmoens]

Stack from ghstack (oldest at bottom):

• -> [CI] Add path-based triggers for niche workflows #3403
• [CI] Separate GPU and CPU tests with pytest markers #3404
• [CI] Add file-based auto-labeling for PR components #3402

Make expensive/niche workflows conditional on labels to reduce CI costs:

• docs.yml: Only builds docs on PRs when Documentation or tutorials/ label
  is present. Always builds on push to main/nightly/release.

• benchmarks_pr.yml: Only runs benchmarks when Performance or
  benchmarks/upload label is present. This is expensive (GPU, runs twice).

• test-linux-sota.yml: Only runs SOTA tests when sota-implementations/
  label is present. Always runs on push to main/nightly/release.

Labels are auto-applied by the file-based labeler (auto-labeler.yml) when
relevant files change, so developers don't need to manually add them.

Core library tests (test-linux.yml) continue to run on all PRs since
components are interconnected (e.g., replay buffers affect collectors).

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/3403

• 📄 Preview Python docs built from this PR

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

✅ You can merge normally! (1 Unrelated Failure)

As of commit 509bf02 with merge base e09ee4f ():

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 Linux / tests-cpu (3.12) / linux-job (gh) (trunk failure)
  test/test_rb.py::TestLazyMemmapStorageCleanup::test_cleanup_registry

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