feat: restructure docs into "chapters" (#12)

* feat(docker, k8s): create containers folder and kubernetes notes

docs/languages/php/dependency-injection.md

@@ -0,0 +1,78 @@
+# Dependency Injection
+
+Explicit definition of a class dependencies with the injection through the constructor or *getters*/*setters*.
+
+```php
+class Foo
+{
+    public function __construct(PDO $pdo)  // depends on PDO
+    {
+        $this->pdo = $pdo;
+    }
+}
+```
+
+## Dependency Injection Container
+
+The **Dependency Injection Container** (DIC) allow to archive all the dependencies in a single `Container` class. Some offer automatic resolution of the dependencies.
+
+## [PHP-DI](https://php-di.org/)
+
+The dependency injection container for humans. Installation: `composer require php-di/php-di`
+
+- **Autowire** functionality: the ability of the container to create and inject the dependency automatically.
+- Use of [Reflection](https://www.php.net/manual/en/intro.reflection.php)
+- Configuration of the container through annotations & PHP code.
+
+```php
+class Foo
+{
+    private $bar;
+    public function __construct(Bar $bar)  // depends on Bar
+    {
+        $this->bar = $bar;
+    }
+}
+
+class Bar{}
+
+$container = new DI\Container();  // DI Container
+$foo = $container->get('Foo');  // get instance of Foo (automatic DI of Bar)
+```
+
+### DIC Configuration
+
+```php
+// Foo.php
+class Foo
+{
+    public function __construct(PDO $pdo)  // depends on PDO
+    {
+        $this->pdo = $pdo;
+    }
+}
+```
+
+```php
+// config.php
+use Psr\Container\ContainerInterface;
+
+// config "primitive" dependencies (dependency => construct & return)
+return [
+    'dsn' => 'sqlite:db.sq3',
+    PDO::class => function(ContainerInterface $c) {
+        return new PDO($c->get('dsn'));
+    },
+
+    ...
+];
+```
+
+```php
+$builder = new \DI\ContainerBuilder();
+$builder->addDefinitions("config.php");  // load config
+$container = $builder->build();  // construct container
+$cart = $container->get(Foo::class);  // Instantiate & Inject
+```
+
+> **Note**: `get("className")` requires the explicit definition of `className` in the config file. `get(ClassName::class)` does not.