[Fleet] Improve Fleet for operator persona

[mostlyjason]

Summary of the problem
Currently, the Fleet app is a mix of use cases for the platform operator and service owner personas. This makes it difficult for the service owner persona because they need to be aware of more advanced concepts. We have a project to move the service owner persona to to the integrations UI #78188. We can then optimize the Fleet app around the platform operator persona and their use cases.

Potential scope, PM will need to better define it:

• Refocus the integrations view on operational use cases for existing integrations like observing health status and fix problems. See Integration Specific Agent and health view and [Fleet] Improve agent observability #78188
• Provide better summary and drill down views on the fleet agents and data steams page to match what we offer on the integrations page
• Surface action items such as agents that need to be updated, unhealthy agents, capacity or performance problems, and more
• Redesign or remove the overview page
• Take enrollment tokens out of the agents page

User stories*

• As a Fleet user, need quick visibility to the health of my fleet so I can take action to fix problems and keep it up to date

List known (technical) restrictions and requirements

Other
PM Lead @mostlyjason
Design lead @hbharding
Collaborators @mukeshelastic