minor #3906 Blockquote introductions (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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Blockquote introductions

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Most component descriptions start with a blockquote introducing the purpose of a particular component. This pull requests adds these introductions where they are missing.

Commits
-------

6d7ef9c unify blockquote introduction for the components

Author: weaverryan

components/config/introduction.rst

@@ -5,12 +5,9 @@
 The Config Component
 ====================
 
-Introduction
-------------
-
-The Config component provides several classes to help you find, load, combine,
-autofill and validate configuration values of any kind, whatever their source
-may be (YAML, XML, INI files, or for instance a database).
+    The Config component provides several classes to help you find, load,
+    combine, autofill and validate configuration values of any kind, whatever
+    their source may be (YAML, XML, INI files, or for instance a database).
 
 .. caution::
 

components/event_dispatcher/introduction.rst

@@ -5,6 +5,10 @@
 The EventDispatcher Component
 =============================
 
+    The EventDispatcher component provides tools that allow your application
+    components to communicate with each other by dispatching events and listening
+    to them.
+
 Introduction
 ------------
 

components/security/introduction.rst

@@ -4,15 +4,12 @@
 The Security Component
 ======================
 
-Introduction
-------------
-
-The Security component provides a complete security system for your web
-application. It ships with facilities for authenticating using HTTP basic
-or digest authentication, interactive form login or X.509 certificate login,
-but also allows you to implement your own authentication strategies.
-Furthermore, the component provides ways to authorize authenticated users
-based on their roles, and it contains an advanced ACL system.
+    The Security component provides a complete security system for your web
+    application. It ships with facilities for authenticating using HTTP basic
+    or digest authentication, interactive form login or X.509 certificate
+    login, but also allows you to implement your own authentication strategies.
+    Furthermore, the component provides ways to authorize authenticated users
+    based on their roles, and it contains an advanced ACL system.
 
 Installation
 ------------

components/stopwatch.rst

@@ -5,7 +5,7 @@
 The Stopwatch Component
 =======================
 
-    Stopwatch component provides a way to profile code.
+    The Stopwatch component provides a way to profile code.
 
 .. versionadded:: 2.2
     The Stopwatch component was introduced in Symfony 2.2. Previously, the