Update NDM Traps and Netflow retention periods in the public doc

[NouhaManai96]

What does this PR do? What is the motivation?

Fixes AGENT-15507
Updates the data retention periods documentation for Network Device Monitoring to accurately reflect:

• NetFlow: Retention can be 15, 30, 60, or 90 days depending on customer plan (previously showed only 30 days)
• SNMP traps: Retention is determined by customer plan with a default of 15 days (SNMP traps are treated as logs behind the scenes, so retention follows Log Management rules)

This addresses inconsistencies between the public documentation and the internal product wiki.

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/nouha/update-ndm-dta-retention-periods-doc/data_security/data_retention_periods