Add docs about timeslice SLI

[dedemorton]

Closes #3580 for stateful docs.

Also updates "Partition by" to "Group by" to match the 8.12 UI.

A couple of thoughts:

• I think we can improve the formatting on this page, but I didn't have time to wrestle with that during this sprint.
• The examples would probably be clearer with screen captures, but the topic is already pretty long. If we plan to add a bunch more indicators, we might want to reconsider how this topic is structured (everything on one page) and consider breaking it down into multiple topics.

[github-actions]

A documentation preview will be available soon:

• 📚 HTML diff
• 📙 Observability Guide

[mergify]

This pull request does not have a backport label. Could you fix it @dedemorton? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[dedemorton]

TODO: Use example suggested by Francismara S:

"Here's one timeslice example i could think of. In this scenario, i'm creating an SLO for a Kubernetes Statefulset, i'm retrieving the last values of both kubernetes.statefulset.replicas.ready and kubernetes.statefulset.replicas.observed. Then, I'm using the equation A == B ? 1 : 0, where A represents the number of ready replicas and B represents the number of observed replicas. This equation returns 1 if the condition A == B is true (indicating the same number of replicas) and 0 otherwise. The condition is when this equation evaluates to a value less than 1."

Screenshot 2024-02-14 at 13.37.31.png