[Cookbook][Dynamic Form Modification] Add AJAX sample
Updates & Fixes after public review
Remove some empty lines from code samples
Shorten AJAX example
feature #3411 [Cookbook][Dynamic Form Modification] Add AJAX sample (bicpi)

This PR was merged into the 2.3 branch.

Discussion
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[Cookbook][Dynamic Form Modification] Add AJAX sample

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | #2464 ("AJAX experience")

Do we need a PHP version of the `create.html.twig` which would mainly contain redundant JavaScript?

Commits
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a75ad9c Shorten AJAX example
ee33dcd Remove some empty lines from code samples
f47a7c3 Updates & Fixes after public review
2533f29 [Cookbook][Dynamic Form Modification] Add AJAX sample