docs: add comprehensive documentation

[marcosgz]

Summary

Adds an 11-file documentation set under docs/:

• Entry points: README.md, getting-started.md, configuration.md
• Core concepts: consumers.md, producers.md, middleware.md
• Runtime: cli.md, supervisor.md, web.md
• Ecosystem: testing.md, rails.md

Covers the full consumer/producer DSL, built-in middlewares, supervisor process model (fork hooks, signal handling, graceful shutdown), the CLI (lepus start, lepus web), and the web dashboard. Includes Rails-specific guidance (Railtie, executor wrapping, Puma plugin).

Test plan

• Skim docs/README.md as a navigation entry point
• Verify consumer/producer DSL examples match the current API
• Verify middleware list in middleware.md matches what ships in the gem
• Verify CLI flags in cli.md match what bundle exec lepus start --help shows
• Verify the signal table in supervisor.md matches actual behavior