Expand discussion of worker health check

[discdiver]

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Describe the issue

The docs don't have much info about worker health checks.

Adding some basic info would be good.

Showing users how to use the worker health check functionality - perhaps in a guide would be nice.

Describe the proposed change

Mention that the default worker webserver port is 8080, but you can change that for the active Prefect profile with the command prefect config set PREFECT_WORKER_WEBSERVER_PORT='8081'.

You can see all the values for the current profile with prefect config view --show-defaults.

Additional context

Start the worker with

prefect worker start --pool 'my-pool' --with-healthcheck

curl http://127.0.0.1:8080/health to check the worker health manually.