docs(examples): add Slurm launch examples

[rhewett-nv]

What does this PR do?

Adds self-contained Slurm launch examples that use the Lens CLI to set resource
attributes, emit launch spans, and write local collector output for plain Slurm
and NVRx workflows.

Issues

N/A

Usage

cd examples/slurm-launch

# Edit account, partition, qos, TEL, CONTAINER_IMAGE, and CONTAINER_MOUNTS first.
sbatch otel_minimal.sbatch
sbatch nvrx_bulk.sbatch
sbatch nvrx_bulk_spare.sbatch
sbatch nvrx_array.sbatch
sbatch nvrx_array_spare.sbatch

Before your PR is "Ready for review"

Pre checks:

• I read the Contributor guidelines.
• All commits are signed off (git commit -s) per the DCO.
• I added or updated tests under tests/ for any behavior change.
• I ran pytest and pre-commit run --all-files locally; both pass.
• I updated docs/ and README.md where behavior, configuration, or public API changed.
• If this changes a lens public API symbol, I updated the cross-repo _fallbacks.py files in Megatron-LM / RL / Gym.

Additional information

Validation on pr6-slurm-launch-examples:

• ../.venv/bin/pre-commit run --all-files: passed
• PYTHONPATH=src ../.venv/bin/python -m pytest -q: 265 passed, 7 warnings
• ../.venv/bin/python -m compileall -q src tests examples/slurm-launch/mock_workload.py: passed
• ../.venv/bin/ruff check examples/slurm-launch/mock_workload.py: passed
• bash -n on all five .sbatch examples: passed
• git diff --check pr5-slurm-resource-attrs-cli..pr6-slurm-launch-examples: passed
• npm --prefix docs/fern run check: passed with 0 errors; one warning because the unauthenticated redirects check is skipped without fern login / FERN_TOKEN

Notes:

• PR6 is one signed commit: 03f9b54 docs(examples): add Slurm launch examples.
• pr6-slurm-launch-examples and schema-v01-resource-attrs point at the same
  commit.
• The PR adds only examples/slurm-launch/ files; it does not change Lens
  runtime code, tests, packaging, docs-site pages, or fallback surfaces.
• Example scripts use nemo-lens set-slurm-resource-attrs --stage sbatch in the
  sbatch context and --stage task inside srun.
• SLUID lookup is done once in the sbatch/script context with sacct; the task
  context inherits LENS_SLURM_SLUID and LENS_SLURM_ARRAY_SLUID.
• The examples use placeholder scheduler/path/image/mount values and the local
  task collector endpoint http://localhost:4317.
• The NVRx examples assume ft_launcher is already installed and on PATH in
  the container.
• The array-spare example may cancel a cold spare before it emits telemetry; that
  is expected for this test shape.

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[ahmadki]

/ok to test 85cc56d