minor #11437 Reorganized the Workflow docs (javiereguiluz)

This PR was squashed before being merged into the 3.4 branch (closes #11437).

Discussion
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Reorganized the Workflow docs

This fixes most of #10747. Main changes:

* The main workflow article + the state machines article have been merged into a theoretical introductory article about workflows and state machines.
* The "usage" article has been deleted and its contents moved to the main workflow article.

------

Missing things:

* We need to solve the (4) idea from #10747, which I don't understand.

Thanks!

Commits
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486b978 Reorganized the Workflow docs

Author: javiereguiluz

_build/redirection_map

@@ -362,3 +362,5 @@
 /console/logging /console
 /frontend/encore/legacy-apps /frontend/encore/legacy-applications
 /contributing/code/patches /contributing/code/pull_requests
+/workflow/state-machines /workflow/introduction
+/workflow/usage /workflow

reference/configuration/framework.rst

@@ -2306,8 +2306,9 @@ type
 
 **type**: ``string`` **possible values**: ``'workflow'`` or ``'state_machine'``
 
-Defines the kind fo workflow that is going to be created, which can be either
-a :doc:`normal workflow </workflow/usage>` or a :doc:`state machine </workflow/state-machines>`.
+Defines the kind of workflow that is going to be created, which can be either
+a normal workflow or a state machine. Read :doc:`this article </workflow/introduction>`
+to know their differences.
 
 .. _`HTTP Host header attacks`: http://www.skeletonscribe.net/2013/05/practical-http-host-header-attacks.html
 .. _`Security Advisory Blog post`: https://symfony.com/blog/security-releases-symfony-2-0-24-2-1-12-2-2-5-and-2-3-3-released#cve-2013-4752-request-gethost-poisoning