Add a visual source-to-artifact showcase to README

[DKFuH]

Why

OrgScript is much easier to understand when people can see one source file and the generated Mermaid and Markdown outputs side by side.

Scope

• improve the README front door with one concise source-to-artifact flow
• show .orgs, Mermaid, and Markdown summary outputs together
• keep the example compact and directly tied to real repo files
• avoid turning the README into a long tutorial

Definition Of Done

• the README contains one clear showcase section

• the showcased source and generated artifacts exist in the repo

• the commands used to generate them are obvious

• the section is scannable on GitHub without extra context

• v0.5.0

• documentation

• dx

• Mermaid and Markdown demo artifacts should already exist and stay current

Favor fast understanding over exhaustiveness. One strong visual flow is better than many weak examples.