Explain error budgets on the stage group dashboards

What does this MR do?

This adds a short explanation on the error budgets visible on the stage group dashboards.

It tries to explain in broad strokes how it is calculated, and links to the project documenting the work that still needs to be done.

In gitlab-com/gl-infra/scalability#952 (closed) we'll be updating the handbook with how these budgets will be used organisationally. This aims to tell developers where the number comes from.

In the future (after gitlab-com/gl-infra/scalability#967 (closed) and gitlab-com/gl-infra/scalability#966 (closed)), we should also add how folks can proceed to investigate where a problem lies. But I don't quite know how that's going to go yet. For now, we only have information on a single component (puma).

Related issues

Related to gitlab-com/gl-infra/scalability#945 (closed), budgets added in gitlab-com/runbooks!3388 (merged)

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