Add clear examples of how to use the V2 client

[cmacknz]

Currently new users of the Elastic agent client are left with looking through the fleet-server and beats code to see real usage examples. We should provide a clear sample usage of the client (perhaps in a toy input) for input developers to use as a starting point.

• Fleet server example usage: https://github.com/elastic/fleet-server/blob/78706a6498e349ea80c19a3fad5610d80209ea4c/internal/pkg/server/agent.go#L103-L119
• Beats example usage: https://github.com/elastic/beats/blob/ce10bc4c84a497b197d67c7b219d358c0392c4ad/x-pack/libbeat/management/managerV2.go#L354-L380

Both of those examples require a new user to know they exist, and understand their usage in the context of the project they implement. The Beats client in particular is the most likely for others to find but also has the additional complexity of generating and reloading Beat configuration files.

[cmacknz]

We need to explain the lifecycle of a unit and how to handle it in code. In particular we should explain what happens when units are added and removed from a policy, what happens when someone edits the input ID of an existing unit in a policy, what the shutdown sequence when all units are stopped in a process is, etc.