This adds docs on how to customize apdex target

What does this MR do?

This MR documents how to add an SLI on the application side. Documentation for !69154 (merged)

The first SLI that's added like this is the request apdex, which allows adjusting the duration target (!69877 (merged)), this also adds documentation on how to do that.

Related issues

• gitlab-com/gl-infra/scalability#1237 (closed)
• gitlab-com/gl-infra/scalability#1238 (closed)

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