[DOCS-15572] Updates for multiple destinations

[maycmlee]

What does this PR do? What is the motivation?

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Modified Files

• https://docs-staging.datadoghq.com/may/update-multiple-destinations/observability_pipelines/configuration/set_up_pipelines
• https://docs-staging.datadoghq.com/may/update-multiple-destinations/observability_pipelines/destinations/

[maycmlee]

Editorial review note: hugo/content/en/observability_pipelines/configuration/set_up_pipelines.md:186 (unchanged by this PR) still says "You can add a total of three destinations for a log pipeline." This contradicts the new 20-destination limit this PR introduces elsewhere — worth updating in the same PR so the doc stays internally consistent.