Add "What gets generated" section to README with example output

[halotukozak]

Description

The README explains how to configure the plugin but doesn't show what the generated code looks like. A "What gets generated" section with a small example (e.g. from a Petstore-like spec) would help potential users understand the output before adopting the library. Should include examples of: a generated data class, an enum, and an API client method.

Acceptance criteria

• README contains a "What gets generated" section
• Shows example of a generated model (data class)
• Shows example of a generated enum
• Shows example of a generated API client method (suspend function)
• Examples are consistent with actual generator output