Add doc comments to public types for library consumers

[raiderrobert]

Context

Public-facing types like GenerateOptions, FullGenerationPlan, GeneratedProject, PlannedFile, GenerationPlan, ResolvedTemplate, TemplateSource, and CachedTemplate have minimal or no doc comments.

Since this crate exposes a library target, users of the library API would benefit from module-level and type-level documentation. For example, PlannedFile has no doc comment explaining the semantics of is_copy vs rendered files.

Suggestion

Add /// doc comments to public types and their key fields, covering:

• What the type represents
• When/how it's used in the pipeline
• Any non-obvious semantics (e.g., is_copy meaning)