minor #12387 Moved "Best Practices" to a dedicated article (javiereguiluz)

This PR was squashed before being merged into the 4.3 branch (closes #12387).

Discussion
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Moved "Best Practices" to a dedicated article

As discussed by the Symfony Docs team, we want to simplify the current "Best Practices" docs into a single article.

The main problem is that the current "Best Practices" explain too many tech details and they duplicated some contents. So, let's move all best practices to a single page and let's describe them with links to the rest of the Symfony Docs.

Commits
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deace0b Moved \"Best Practices\" to a dedicated article

Author: javiereguiluz

_build/redirection_map

@@ -460,3 +460,15 @@
 /testing/doctrine /testing/database
 /doctrine/lifecycle_callbacks /doctrine/events
 /doctrine/event_listeners_subscribers /doctrine/events
+/best_practices/index /best_practices
+/best_practices/introduction /best_practices
+/best_practices/creating-the-project /best_practices
+/best_practices/configuration /best_practices
+/best_practices/business-logic /best_practices
+/best_practices/controllers /best_practices
+/best_practices/templates /best_practices
+/best_practices/forms /best_practices
+/best_practices/i18n /best_practices
+/best_practices/security /best_practices
+/best_practices/web-assets /best_practices
+/best_practices/tests /best_practices