Docs: Add dashboard generation process and usage guidance

What does this MR do?

Clarify the Grafana dashboard generation and import process for customers implementing GitLab Runner monitoring in their environments.

Customers implementing GitLab Runner dashboards were unclear about:

• How to convert jsonnet source files to JSON format for Grafana import
• The step-by-step process from source to deployment
• How to customize template variables for their environments
• What each dashboard is used for

Related issues

https://gitlab.com/ubs-group1/ubs/ubs-global/-/issues/355

Author's checklist

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Reviewer's checklist

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