minor #1574 Docs: Add documentation how to use RoutingServiceProvider (Yanni)

This PR was merged into the 2.2.x-dev branch.

Discussion
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Docs: Add documentation how to use RoutingServiceProvider

Silex is now on 2.0 and there is zero documentation about [URL Generator](https://silex.symfony.com/doc/1.3/providers/url_generator.html) which became part of RoutingServiceProvider

Commits
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85ce3f09 Docs: Add documentation how to use RoutingServiceProvider

doc/providers/index.rst

@@ -22,3 +22,4 @@ Built-in Service Providers
     service_controller
     var_dumper
     doctrine
+    routing

doc/providers/routing.rst

+Routing
+=======
+
+The *RoutingServiceProvider* provides a service for generating URLs for
+named routes.
+
+Parameters
+----------
+
+None.
+
+Services
+--------
+
+* **url_generator**: An instance of `UrlGenerator
+  <http://api.symfony.com/master/Symfony/Component/Routing/Generator/UrlGenerator.html>`_,
+  using the `RouteCollection
+  <http://api.symfony.com/master/Symfony/Component/Routing/RouteCollection.html>`_
+  that is provided through the ``routes`` service. It has a ``generate``
+  method, which takes the route name as an argument, followed by an array of
+  route parameters.
+
+Registering
+-----------
+
+.. code-block:: php
+
+    $app->register(new Silex\Provider\RoutingServiceProvider());
+
+Usage
+-----
+
+The Routing provider provides a ``url_generator`` service::
+
+    $app->get('/', function () {
+        return 'welcome to the homepage';
+    })
+    ->bind('homepage');
+
+    $app->get('/hello/{name}', function ($name) {
+        return "Hello $name!";
+    })
+    ->bind('hello');
+
+    $app->get('/navigation', function () use ($app) {
+        return '<a href="'.$app['url_generator']->generate('homepage').'">Home</a>'.
+               ' | '.
+               '<a href="'.$app['url_generator']->generate('hello', array('name' => 'Igor')).'">Hello Igor</a>';
+    });
+
+When using Twig, the service can be used like this:
+
+.. code-block:: jinja
+
+    {{ app.url_generator.generate('homepage') }}
+
+Moreover, if you have ``twig-bridge`` as a Composer dep, you will have access
+to the ``path()`` and ``url()`` functions:
+
+.. code-block:: jinja
+
+    {{ path('homepage') }}
+    {{ url('homepage') }} {# generates the absolute url http://example.org/ #}
+    {{ path('hello', {name: 'Fabien'}) }}
+    {{ url('hello', {name: 'Fabien'}) }} {# generates the absolute url http://example.org/hello/Fabien #}
+
+Traits
+------
+
+``Silex\Application\UrlGeneratorTrait`` adds the following shortcuts:
+
+* **path**: Generates a path.
+
+* **url**: Generates an absolute URL.
+
+.. code-block:: php
+
+    $app->path('homepage');
+    $app->url('homepage');