Update GLQL pipeline analytics docs for the new pipelines engine

What does this MR do?

Updates the GLQL pipeline analytics documentation for the migration from the finishedPipelines aggregation engine to the new pipelines engine (glql!416 (merged)). The change takes effect with GitLab 19.2.

Specifically:

• The status filter now accepts all 13 CI/CD pipeline statuses (previously only canceled, failed, skipped, and success).
• Analytics mode now covers pipelines in all states, including in-progress pipelines, instead of finished pipelines only.
• totalCount counts all pipelines, including in-progress ones.
• The rate metrics (successRate, failureRate, canceledRate, and skippedRate) are calculated among completed pipelines only, and return null when no completed pipelines match the query.

Related issues

• glql!416 (merged)
• glql#149 (closed)

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