feat: richer init scaffolding with feature demos #142

Problem

`chronicle init` creates a minimal scaffold: one `chronicle.yaml` and one `index.mdx` with basic frontmatter. New users don't see what Chronicle can do — callouts, code blocks, mermaid diagrams, API docs, versioning, etc.

Suggested Improvement

• Add 2-3 sample pages demonstrating key features:
  • A "Getting Started" page with callouts, code blocks, and a table
  • A "Components" page showing mermaid diagrams, admonitions, and images
• Optionally scaffold with `--with-api` flag to include a sample OpenAPI spec
• Add comments in the generated `chronicle.yaml` explaining each config section