[cmd/mdatagen] Allow to show more information related to component warnings

[atoulme]

As exposed in https://github.com/open-telemetry/opentelemetry-collector-contrib/pull/32130, component authors now document explicitly that an empty warnings entry is to be considered as having reviewed warnings associated with the component.

Right now, this doesn't translate to a README change. It would be good to have a way to explicitly call the state of warning maturation for a component:

• Not reviewed
• Reviewed, warnings identified
• Reviewed, no warnings

[SpencerTorres]

My suggestion based on your points:

1. default/undefined value can be "Not reviewed"
2. warnings: [...] can be "Reviewed, warnings identified"
3. warnings: [] can be "Reviewed, no warnings"

The README should generate with placeholder text for cases 1 and 2.

[atoulme]

The problem is that the default value is [].