Add interactive Phoenix LiveView demo

[ivan-podgurskiy]

Summary

• Add a standalone LiveView playground covering all nine Humanizer functions, options, presets, validation, and generated example code.
• Add production releases, Docker deployment, health checks, and documentation.
• Extend CI to test/build the demo and ensure it remains excluded from the Hex package.
• Add comprehensive playground and LiveView tests and refine the demo’s styling and branding.

Test plan

• cd demo && mix format --check-formatted
• mix compile --warnings-as-errors
• mix test
• MIX_ENV=prod mix assets.deploy
• MIX_ENV=prod mix release
• docker build -f demo/Dockerfile -t humanizer-demo .