[Fleet] Improve onboarding on Agents page for ESS/ECE

[mostlyjason]

Currently, when a user visits the Agents page after starting a new deployment, its not obvious what step they need to take next. They see the Elastic Cloud agent, but they aren't familiar with what that is yet. Also, they can't use it for anything because we don't allow other integrations to run there. Previously, we had an empty state screen making it obvious that users need to add an agent, but that is not visible on ESS/ECE.

When a user visits this page for the first time, can we offer a short intro to Fleet & Elastic Agent, and make it more obvious that the next step is adding an agent? Perhaps something like this?

Welcome to Fleet! To get started, [add an Elastic Agent] to a server, laptop or other host. When you enroll an agent into Fleet, you can see its status and update it here. You can then add integrations to monitor your host or protect it from security threats.

Current page:

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)