[Form] Revamp Symfony Form docs

[javiereguiluz]

• [Form] Merge some small articles into the main form article #21740
• [Form] Explain more basic concepts about Symfony forms #21743
• [Form] Revamp the form events docs #21750
• [Form] Revamp the docs about form theming and customization #21785
• ...
• ...

[lacatoire]

@javiereguiluz is this still open ? what would you want more in an issue ?

[94noni]

can be great to include also the form flow feature and the ux related form feature
as a more global entry point for all forms related in sf ecosystem