Remove header from Datadog Agent shortcode and add to docs

[maycmlee]

What does this PR do? What is the motivation?

Merge instructions

Removes the header from the Datadog Agent shortcode and adds it to the markdown docs.

• [x] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/may/op-remove-ddagent-header-shortcode/observability_pipelines/archive_logs/datadog_agent
• https://docs-staging.datadoghq.com/may/op-remove-ddagent-header-shortcode/observability_pipelines/dual_ship_logs/datadog_agent
• https://docs-staging.datadoghq.com/may/op-remove-ddagent-header-shortcode/observability_pipelines/log_volume_control/datadog_agent
• https://docs-staging.datadoghq.com/may/op-remove-ddagent-header-shortcode/observability_pipelines/sensitive_data_redaction/datadog_agent
• https://docs-staging.datadoghq.com/may/op-remove-ddagent-header-shortcode/observability_pipelines/split_logs/datadog_agent