Add hello-world example to examples directory

[Copilot]

Adds a minimal, self-contained example to reduce onboarding friction and provide a quick sanity-check for new users.

Changes

• examples/hello-world/hello.md – sample Markdown input covering common formatting (headings, bold, code block)
• examples/hello-world/renderflow.yaml – minimal config wiring hello.md → HTML output under dist/
• examples/hello-world/README.md – prerequisites, run instructions, expected output path, and dry-run usage

Usage

cd examples/hello-world
renderflow build
# → dist/hello.html

[devactivity-app]

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