docs: refresh air-gapped values example

[omribz156]

Summary:

• Replace the old v1.7.0 raw values.yaml fetch with the Helm chart values flow used elsewhere in the GPU Operator docs.
• Remove fixed GPU Operator v1.9-era component tags from the air-gapped sample and show a version-neutral local registry override pattern.
• Add guidance to keep image and version values from the selected chart's values.yaml.

Verification:

• rg -n "v1\.7\.0|v1\.9\.0|11\.4\.2-base-ubi8|v0\.10\.0-ubi8|2\.3\.1-2\.6\.0|helm show values|Sample ``values.yaml``|<driver-version>" gpu-operator/install-gpu-operator-air-gapped.rst
• git diff --check

Notes:

• I could not run pre-commit or helm locally because those CLIs are not installed in this environment.
• The helm show values flow is already used in the current GPU Operator docs (getting-started.rst and upgrade.rst).

Addresses part of #391.

This was implemented with Codex assistance, with the final diff manually reviewed and kept to the docs page scope.