Document service overview comparison

[bmorelli25]

Update documentation to reflect the new APM Service Overview comparison.

The service overview introduces a new view for the selected service that encompasses a lot of the service data from existing views like metrics, transactions, and errors. The primary goal of the view is to aid in troubleshooting a service from a birds eye view rather than immediately diving into the Transactions, as that's the current landing page for a selected service. There's more information about the user stories and goals of the view in the design issue linked above.

The general idea of the time-series comparison is to provide the user with more background on the currently selected time range. It's typically part of the analysis to investigate how the service was behaving a week ago or similar to give an indication of whether something has been slowly increasing or it's a sudden spike.

We want to make it simple for our users to select a time range and immediately have a comparison on the charts as a visual aid. This means adding an option next to the existing time range selection that allows the user to turn on/off a comparison time-series plot on all the relevant time-series charts.

See elastic/kibana#81147 for more information.