This pull request includes significant updates to the Lennarb framework, focusing on enhancing the plugin system, improving documentation, and making various codebase refinements. The most important changes include the introduction of a new plugin system, comprehensive updates to the getting started guide, and various codebase improvements for better maintainability and functionality.

Plugin System Enhancements:

• guides/plugin/readme.md: Expanded the plugin system documentation with core features, plugin creation, usage, and best practices.
• lib/lennarb.rb: Implemented a modular and thread-safe plugin system with methods for registering, loading, and configuring plugins.

Documentation Updates:

• guides/getting-started/readme.md: Overhauled the getting started guide to provide a comprehensive introduction to Lennarb, including installation, basic usage, application structure, routing, middleware support, and best practices.

Codebase Improvements:

• lennarb.gemspec: Removed the puma development dependency to streamline the gemspec file.
• .rubocop.yml: Cleaned up the RuboCop configuration by removing unnecessary indentation rules.

Changelog Update:

• changelog.md: Documented the new features, changes, and removals in version 1.3.0, including the introduction of the Lennarb::Plugin module and changes to the Lennarb class.

Minor Fixes:

• guides/response/readme.md: Updated the method for setting response headers to use res['Content-Type'] instead of res.headers['Content-Type'].