Add `generateKdoc` configuration option to Gradle plugin

[halotukozak]

Description

Add a generateKdoc (Boolean, default true) option to JustworksSpecConfiguration. When set to false, generated code should not contain any KDoc comments. Useful for reducing generated file size and noise.

Acceptance criteria

• JustworksSpecConfiguration DSL supports generateKdoc property
• generateKdoc = false suppresses all KDoc in generated client and model code
• Default is true — no breaking change
• Plugin functional test covers both values
• README documents the option