minor #17997 Updated all the README files (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #17997).

Discussion
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Updated all the README files

| Q             | A
| ------------- | ---
| Branch        | 2.3
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Related to #17995.

Commits
-------

2e81248 Updated all the README files

Author: fabpot

README.md

@@ -1,21 +1,13 @@
 Yaml Component
 ==============
 
-YAML implements most of the YAML 1.2 specification.
-
-```php
-use Symfony\Component\Yaml\Yaml;
-
-$array = Yaml::parse(file_get_contents(filename));
-
-print Yaml::dump($array);
-```
+The Yaml component loads and dumps YAML files.
 
 Resources
 ---------
 
-You can run the unit tests with the following command:
-
-    $ cd path/to/Symfony/Component/Yaml/
-    $ composer install
-    $ phpunit
+  * [Documentation](https://symfony.com/doc/current/components/yaml/index.html)
+  * [Contributing](https://symfony.com/doc/current/contributing/index.html)
+  * [Report issues](https://github.com/symfony/symfony/issues) and
+    [send Pull Requests](https://github.com/symfony/symfony/pulls)
+    in the [main Symfony repository](https://github.com/symfony/symfony)