docs: rewrite README with professional project documentation

[syntaxmage05]

Motivation

• Replace the generated/HTML-heavy README with a concise, professional document that clearly onboards developers and contributors.
• Provide explicit local setup and operational guidance so new maintainers can get the app running quickly.
• Surface runtime behavior (background jobs, core routes, and common commands) to reduce friction when debugging or deploying.

Description

• Rewrote README.md, removing HTML and noisy badge markup and replacing it with structured Markdown sections including Overview, Features, Tech Stack, Prerequisites, Local Setup, Useful Commands, Background Jobs, Core Routes, Project Structure, Deployment, and Contributing.
• Added clear local setup steps and examples for developer workflows including bundle install, bin/rails db:prepare, and bin/dev, and documented what bin/dev launches (web server, Tailwind watcher, Sidekiq worker).
• Documented recurring Sidekiq Cron jobs (Reminders::FindTeamsJob and Recaps::FindTeamsJob) and the main application routes (/, /t, /s, /sidekiq).
• Simplified onboarding content and provided useful commands for testing and static analysis such as bundle exec rspec, bin/brakeman, and bin/rubocop.

Testing

• Attempted to run bundle exec rspec --dry-run to verify test command availability, which failed because the rspec executable is not available in this environment (project dependencies are not installed).
• No other automated test runs were executed in this environment.

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