Fill documentation gaps: tutorials, template author guide, and architecture diagram

[ericof]

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Summary

The documentation scaffolding (#136) created placeholder pages, but key content is still missing. Several reference docs exist, but how-to guides are stubs and there is no architecture overview.

Scope

Priority 1: Getting-started tutorial

A complete walkthrough for first-time users: install, run, answer prompts, inspect output. Should cover uvx cookieplone for the common case.

Priority 2: Template author guide

How to create a new template, use computed fields, add validators, structure sub-templates, and test locally.

Priority 3: Architecture diagram

A visual overview of the data flow: CLI → repository resolution → config/state (v1/v2 schemas) → wizard (tui-forms) → generator (cookiecutter) → hooks. This helps both contributors and template authors understand where their code fits.

Acceptance criteria

• Getting-started tutorial is complete and renders correctly
• Template author guide covers schema v2, computed fields, validators, and sub-templates
• Architecture diagram (Mermaid or similar) added to concepts section
• All new docs pass make docs-vale linting