minor #9548 New section about JSON file manifest strategy (jeremyjumeau, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #9548).

Discussion
----------

New section about JSON file manifest strategy

@see https://symfony.com/blog/new-in-symfony-3-3-manifest-based-asset-versioning

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

b8a67a4 Added a missing comma
3266473 Reword
b33168a Link to the asset component documentation
e0ed723 Fix typo
3ef51fc New section about JSON file manifest strategy

Author: javiereguiluz

components/asset.rst

@@ -139,6 +139,33 @@ string as the second argument of the ``StaticVersionStrategy`` constructor::
     echo $package->getUrl('image.png');
     // result: v1/image.png
 
+JSON File Manifest
+..................
+
+A popular strategy to manage asset versioning, which is used by tools such as
+`Webpack`_, is to generate a JSON file mapping all source file names to their
+corresponding output file:
+
+.. code-block:: json
+
+    // rev-manifest.json
+    {
+        "css/app.css": "build/css/app.b916426ea1d10021f3f17ce8031f93c2.css",
+        "js/app.js": "build/js/app.13630905267b809161e71d0f8a0c017b.js",
+        "...": "..."
+    }
+
+In those cases, use the 
+:class:`Symfony\\Component\\Asset\\VersionStrategy\\JsonManifestVersionStrategy`::
+
+    use Symfony\Component\Asset\Package;
+    use Symfony\Component\Asset\VersionStrategy\JsonManifestVersionStrategy;
+
+    $package = new Package(new JsonManifestVersionStrategy(__DIR__'./rev-manifest.json'));
+
+    echo $package->getUrl('css/app.css');
+    // result: build/css/app.b916426ea1d10021f3f17ce8031f93c2.css
+
 Custom Version Strategies
 .........................
 
@@ -349,3 +376,4 @@ Learn more
 ----------
 
 .. _Packagist: https://packagist.org/packages/symfony/asset
+.. _`Webpack`: https://webpack.js.org/

frontend/encore/versioning.rst

@@ -59,3 +59,8 @@ like normal:
     <script src="{{ asset('build/app.js') }}"></script>
 
     <link href="{{ asset('build/dashboard.css') }}" rel="stylesheet" />
+
+Learn more
+----------
+
+* :doc:`/components/asset`