[Uptime]: Document that Uptime UI is only shown if Heartbeat data is available

[dedemorton]

Description

Based on this info from @bmorelli25, I think we should update the Uptime documentation so users who don't have Heartbeat data will understand why they can't find Uptime in the UI:

By default, Uptime is no longer shown if users have no Heartbeat data, because the preferred approach is to use the Synthetics Kibana app. Synthetics provides a better experience for managing monitors than Uptime (both through the UI, and/or as project monitors.
All new users have it turned off by default but it’s still there to turn on. Existing users will continue to see Uptime.

Resources

Not sure

Which documentation set does this change impact?

Stateful only

Feature differences

n/a

What release is this request related to?

N/A

Collaboration model

The documentation team

Point of contact.

Main contact: unknown

Stakeholders:

[dedemorton]

IMO the notice at the top of the intro page is not explicit enough and might be missed by folks who enter the content lower down in the hierarchy.