docs: use helm pull for air-gapped chart downloads

[efegokdemir]

Summary

Updates the air-gapped GPU Operator installation guide to use Helm's current pull command when downloading the chart from the NGC repository.

This addresses #391 by correcting the stale chart-download example on the air-gapped installation page. No existing open PR covers this exact change.

Changes

• Replace the deprecated helm fetch example with helm pull.
• Update the surrounding instruction to match the command.

Testing

• git diff --check — passed.
• Static reStructuredText command/link review — passed.
• Full Sphinx build — not run because sphinx-build is not installed locally.

Notes

The change is limited to the air-gapped installation guide; the chart URL, version placeholder, and subsequent installation command remain unchanged.

AI assistance was used during source inspection and drafting. I reviewed the complete diff and take responsibility for the submitted change.