[DOCS-14692] Add Splunk HEC metrics destination doc

[maycmlee]

What does this PR do? What is the motivation?

• Adds multi-lang code for Splunk HEC destination logs and metrics
• Updates the nav
• Adds a shortcode for secrets defaults

Merge readiness

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/may/splunk-hec-metrics/observability_pipelines/destinations/splunk_hec/
• https://docs-staging.datadoghq.com/may/splunk-hec-metrics/observability_pipelines/destinations/splunk_hec/metrics

Renamed files

• https://docs-staging.datadoghq.com/may/splunk-hec-metrics/observability_pipelines/destinations/splunk_hec/logs

Modified Files

• https://docs-staging.datadoghq.com/may/splunk-hec-metrics/observability_pipelines/destinations/