minor #12114 Revamped the routing documentation (javiereguiluz)

This PR was squashed before being merged into the 4.3 branch (closes #12114).

Discussion
----------

Revamped the routing documentation

The Symfony Docs team has decided to reorganize all the routing articles and create a single reference article explaining all the common routing features.

This PR merges several small files into the main `/routing.rst` article and I'll merge other small articles very soon. Meanwhile you can start reviewing these changes. Thank you!

Commits
-------

1268ba7 Revamped the routing documentation

Author: javiereguiluz

_build/redirection_map

@@ -429,3 +429,17 @@
 /setup/composer /setup
 /security/security_checker /setup
 /service_container/parameters /configuration
+/routing/generate_url_javascript /routing
+/routing/slash_in_parameter /routing
+/routing/scheme /routing
+/routing/optional_placeholders /routing
+/routing/conditions /routing
+/routing/requirements /routing
+/routing/redirect_trailing_slash /routing
+/routing/debug /routing
+/routing/service_container_parameters /routing
+/routing/redirect_in_config /routing
+/routing/external_resources /routing
+/routing/hostname_pattern /routing
+/routing/extra_information /routing
+/console/request_context /routing