docs: standardize README following org template

[phmatray]

Summary

• Replaces the minimal one-line README with a comprehensive, template-compliant document
• Covers badge wall, problem/solution, features checklist, tech stack, getting started, usage examples with real code from the codebase, architecture diagram, project structure, and roadmap
• Accurately reflects the project's current state as an early-stage (v0.1.0) library bridging Blazor SSR with HTMX via FastEndpoints

Test plan

• Verify all badge URLs resolve correctly on GitHub
• Confirm NuGet badge links to the correct package
• Review code examples for accuracy against current source
• Check that all internal links (TOC anchors) work