New documentation for observability

Observability on Depot isn't new, but the docs for it are! We added new documentation to help you monitor and analyze your Depot builds and workflows:

• Organization-level view: The observability overview summarizes usage metrics, storage breakdowns, and health indicators across all Depot products.
• Container builds: Build metrics and build logs to track build duration, cache performance, resource utilization, and debug specific steps.
• GitHub Actions: Metrics and analytics and job logs to monitor workflow performance and search across all jobs.