Update BYOC Logs sizing tier resources

[shuheiktgw]

What does this PR do? What is the motivation?

Updates the BYOC Logs Helm chart sizing tier documentation to:

• explain what podSize configures and identify xlarge as the default;
• document the Kubernetes system, DaemonSet, and add-on reservations used to calculate pod resources;
• show both nominal and actual CPU and memory requests for every sizing tier;
• clarify that the presets omit CPU limits while setting equal memory requests and limits.

This keeps the public sizing guide aligned with the updated CloudPrem Helm chart sizing map.

Merge readiness

• Ready for merge

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Additional notes

• Related Helm chart PR: https://github.com/DataDog/pomsky-helm-charts/pull/104
• Validation: git diff --check

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/shuheiktgw/update-byoc-sizing-tiers/byoc-logs/operate/sizing