gpu: document required system-probe.yaml flag and gpu.enabled for non-containerized Linux hosts

[gouri-yerra]

Summary

• Fixes a documentation gap in the GPU integration README where the Linux (non-containerized) Host setup section was missing gpu.enabled: true from the datadog.yaml block and did not clearly communicate that system-probe.yaml must also be configured
• Restructures the Host configuration section into numbered steps to make it unambiguous that both datadog.yaml and system-probe.yaml require changes
• Adds an explicit note that gpu_monitoring.enabled: true in system-probe.yaml is required even for non-containerized hosts

Motivation

Tracked in AGENT-16266. A customer (Norfolk Southern Corp) followed the official setup docs and the shipped datadog.yaml.example/system-probe.yaml.example templates, neither of which document the system-probe.yaml flag. The result was system-probe running healthy (eBPF loaded, T4 discovered, uprobes attached) while the core-agent GPU check never instantiated (Last Check from Core Agent: 1969-12-31). The integrations-core README already covered both files but the steps were easy to miss — this PR makes them explicit and sequential.

Test plan

• Verify the rendered README displays the three-step Host configuration correctly
• Confirm gpu.enabled: true now appears in the datadog.yaml code block
• Confirm gpu_monitoring.enabled: true and the system-probe.yaml step are clearly presented alongside the datadog.yaml step

🤖 Generated with Claude Code

[buraizu]

Thanks for the PR, just requesting a few minor edits for style and consistency. Let me know if you have any questions!

[gouri-yerra]

1. Removed bold from both → both
2. Step 1: → 1. (numbered list)
3. Step 2: Add the following parameters → 2. Add the following parameter (singular, since it's just gpu_monitoring.enabled)
4. Step 3: → 3.
5. The check is also enabled... However, it can also → The check is enabled... The check can also

[dd-octo-sts]

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