Improve developer documentation

[rtcoder]

Changes

• Rebuild docs/index.html into a Laravel-like developer guide with persistent navigation and grouped sections.
• Document installation, configuration, directory structure, request lifecycle, routing, controllers, requests, responses, validation, middleware, modules, service container, commands, database, query builder, models, migrations, logging, cache, doctor, testing, and releases.
• Add light and dark documentation themes with a sidebar toggle, system preference fallback, and stored user preference when browser storage is available.
• Add responsive documentation layout handling for code blocks, tables, and narrow screens.
• Mark the developer documentation milestone as completed in REFACTORING.md.

Verification

• composer validate --no-check-publish
• composer test
• Documentation anchor check for all in-page navigation links
• Browser check at /docs/index.html on desktop and mobile viewport
• Browser check for switching between light and dark documentation themes

Release

• Version label: v0.20.0