[TypeInfo] Add documentation

components/type_info.rst

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+The TypeInfo Component
+======================
+
+    The TypeInfo component extracts PHP types information. It aims to:
+
+    - Have a powerful Type definition that can handle union, intersections, and generics (and could be even more extended)
+
+    - Being able to get types from anything, such as properties, method arguments, return types, and raw strings (and can also be extended).
+
+.. caution::
+
+    This component is :doc:`experimental </contributing/code/experimental>` and could be changed at any time
+    without prior notice.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/type-info
+
+.. include:: /components/require_autoload.rst.inc
+
+Usage
+-----
+
+This component will gives you a :class:`Symfony\\Component\\TypeInfo\\Type` object that represents
+the PHP type of whatever you builded or asked to resolve.
+
+There are two ways to use this component. First one is to create a type manually thanks
+to :class:`Symfony\\Component\\TypeInfo\\Type` static methods as following::
+
+    use Symfony\Component\TypeInfo\Type;
+
+    Type::int();
+    Type::nullable(Type::string());
+    Type::generic(Type::object(Collection::class), Type::int());
+    Type::list(Type::bool());
+    Type::intersection(Type::object(\Stringable::class), Type::object(\Iterator::class));
+
+    // Many others are available and can be
+    // found in Symfony\Component\TypeInfo\TypeFactoryTrait
+
+
+Second way to use TypeInfo is to resolve a type based on reflection or a simple string::
+
+    use Symfony\Component\TypeInfo\Type;
+    use Symfony\Component\TypeInfo\TypeResolver\TypeResolver;
+
+    // Instantiate a new resolver
+    $typeResolver = TypeResolver::create();
+
+    // Then resolve types for any subject
+    $typeResolver->resolve(new \ReflectionProperty(Dummy::class, 'id')); // returns an "int" Type instance
+    $typeResolver->resolve('bool'); // returns a "bool" Type instance
+
+    // Types can be instantiated thanks to static factories
+    $type = Type::list(Type::nullable(Type::bool()));
+
+    // Type instances have several helper methods
+    $type->getBaseType() // returns an "array" Type instance
+    $type->getCollectionKeyType(); // returns an "int" Type instance
+    $type->getCollectionValueType()->isNullable(); // returns true
+
+Each of this rows will return you a Type instance that will corresponds to whatever static method you used to build it.
+We also can resolve a type from a string like we can see in this example with the `'bool'` parameter it is mostly
+designed that way so we can give TypeInfo a string from whatever was extracted from existing phpDoc within PropertyInfo.
+
+.. note::
+
+    To support raw string resolving, you need to install ``phpstan/phpdoc-parser`` package.