[docs]: module schema docs should highlight required keys in a common way per config module

# Documentation request

To cut down on unnecessary round trips trying to deploy with invalid cloud-config user-data, ensure all **required** keys are represented in config module documentation in a standard way.

Without such guidance, it's very costly to deploy a VM, discover warnings/errors from logs for any missing required keys and have to debug that on a failed VM launch with `sudo cloud-init schema --system --annotate` or searching the static JSON schema directly to discover the missed required keys

Some config modules document Optional/required keys in the description of the key, but it would be ideal to define a standard markup for such keys.