docs: add plugin guides

guides/plugin/readme.md

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+# Plugin System
+
+## Overview
+
+The Lennarb Plugin System allows you to extend the functionality of your Lennarb application by registering and loading plugins. This system is designed to be simple and flexible, enabling you to add custom behaviors and features to your application effortlessly.
+
+## Implementation Details
+
+The plugin system is implemented using a module, `Lennarb::Plugin`, which centralizes the registration and loading of plugins.
+
+### Module: `Plugins`
+
+```ruby
+class Lennarb
+	module Plugins
+		@plugins = {}
+
+		def self.register_plugin(name, mod)
+			@plugins[name] = mod
+		end
+
+		def self.load_plugin(name)
+			@plugins[name] || raise("Plugin #{name} did not register itself correctly")
+		end
+
+		def self.plugins
+			@plugins
+		end
+	end
+end
+```
+
+## Usage
+
+### Registering a Plugin
+
+To register a plugin, define a module with the desired functionality and register it with the Plugin module.
+
+```ruby
+module MyCustomPlugin
+  def custom_method
+    "Custom Method Executed"
+  end
+end
+
+Plugins.register(:my_custom_plugin, MyCustomPlugin)
+```
+
+### Load and Use a Plugin
+
+To load and use a plugin in your Lennarb application, call the plugin method in your application class.
+
+```ruby
+class MyApp < Lennarb::Application::Base
+  plugin :my_custom_plugin
+
+  get '/custom' do |_req, res|
+    res.status = 200
+    res.html(custom_method)
+  end
+end
+```
+
+In this example, the custom_method defined in `MyCustomPlugin` is available in the routes of `MyApp`.
+
+## Conclusion
+
+The Lennarb Plugin System provides a simple and flexible way to extend your application's functionality. By registering and loading plugins, you can easily add custom behaviors and features to your Lennarb application.
+