[Product Analytics] Update funnel conversion calculation

[zhangmatthieu]

What does this PR do? What is the motivation?

Updates the funnel analysis documentation to describe the entry-based conversion model. It explains that each entry starts an independent attempt, shared final-step events count toward the earliest entry, and unrelated events do not affect conversion.

This aligns the conversion example with the behavior shown in Product Analytics while preserving the existing examples and tone.

Merge readiness

• Ready for merge

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AI assistance

Used Codex to audit the existing page against the updated funnel behavior, draft the wording changes, and run the documentation style checks.

Additional notes

Validation:

• git diff --check
• Vale 3.9.1 on the changed Markdown file; no findings on the edited lines

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/matthieu.zhang/update-funnel-conversion-model/product_analytics/charts/funnel_analysis