[doc] Improvements to ScalaForms

General rewrite of ScalaForms.

* Added a list of Forms mapping.
* Added list of helpers.
* Add diagram showing form handling flow (using draw.io format)
* Merged ScalaFormHelpers.md into ScalaForms to show helper usage directly with the form mapping.
* Added a ScalaCustomValidations.md file to show how to create custom constraints.
* Moved the Custom Field Validations section out to its own file.
* Added links for everything.