Reorganise and add documentation on new stage group dashboards

What does this MR do?

This splits up the documentation that used to live in doc/development/stage_group_dashboards.md into 4 documents:

• development/stage_group_observability/index.md: This explains how we split our metrics to provide views specific to stage groups and explains error budget: new intro, minor tweaks to the error budget content.
• development/stage_group_observability/dashboards/index.md: This explains where to find dashboards and some general things on how to use Grafana: mostly moved from the old documentation.
  • development/stage_group_observability/dashboards/stage_group_dashboard.md: This explains how to read and customise the stage group dashboard: mostly moved from the old documentation.
  • development/stage_group_observability/dashboards/error_budget_detail.md: This is new documentation explaining how to read the new dashboard.

Related issues

For gitlab-com/gl-infra/scalability#1587 (closed)

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