feature #5456 Completely re-reading the data transformers chapter (weaverryan)

This PR was merged into the 2.3 branch.

Discussion
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Completely re-reading the data transformers chapter

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #5307

**Note**: `setDefaultOptions()` -> `configureOptions()` in 2.7 after merge

Hi guys!

This follows after #5307, which helped update the article (but also added some new complexity). Data transformers are super cool, but I think we've over-complicated them in the past. Even for me, before re-writing this, I tried to avoid transformers because I thought they were hard :).

The code for this can be found here: https://github.com/weaverryan/docs-data-transformer/commits/finish, where each commit that starts with "[tuts-apply-diff]" is a "step" in the tutorial.

My goals:

1) Show an easy example using the CallbackTransformer in the beginning
2) Remove complexity related to any factories that were there before
3) Moved theoretical section to the bottom

Thanks!

Commits
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0762848 Adding an example of how the other format for attaching transformers looks
28d7f89 fixed typo thanks to @OskarStark
41bb907 adding use statement
1514fdd using the aliases entity manager service name
9be07f0 Many tweaks thanks to review
b48feda Completely re-reading the data transformers chapter:

Author: weaverryan

book/forms.rst

@@ -1141,6 +1141,8 @@ the choice is ultimately up to you.
 
         $form->get('dueDate')->setData(new \DateTime());
 
+.. _form-as-services:
+
 Defining your Forms as Services
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~