[Form] Add debug:form command

[javiereguiluz]

See symfony/symfony#23694. We need to mention this new command in the main debugging article.

[yceruto]

Is it normal that http://symfony.com/doc/current/debug.html is empty?

[javiereguiluz]

See also: symfony/symfony#24185

[ogizanagi]

And symfony/symfony#24208 ;) (but not merged yet)

[javiereguiluz]

Fixed by #8517 ... we can create a new issue/PR if we want to improve the mention to this new command. Thanks.