Consolidate DORA docs

What does this MR do and why?

Originally, DORA metrics were introduced as part of CI/CD analytics and only later we added the APIs the tiles to VSA and Insights. As a result, DORA capabilities are now spread across multiple platform pages. Following the new category for DORA, we need to consolidate our DORA docs in one page.

This related to Add new DORA Metrics category (gitlab-com/www-gitlab-com!111590 - merged)

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