docs: make the repository role unmistakable

[DavidWilliamBalbino]

Turn the README into an intentional product or migration page.

• explain what the repository owns and what stays outside its boundary
• lead with concrete use cases instead of a repeated installer block
• provide a short, contextual quick start and canonical documentation links
• clearly label compatibility packages and Composer distribution mirrors
• preserve detailed API, architecture, production, and limitation material

Documentation-only change; product-page structure and git diff --check were validated locally.