Started to write docs about the form subsystem

docs/usage/forms.md

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+---
+title: Forms
+layout: default
+parent: Usage
+---
+
+# Forms
+
+The settings-bundle comes with an integration with symfony/forms, which allows you to easily create forms for your settings classes. The fields of the form, their form type and options are automatically derived from the settings metadata like the property types and attributes. This way allows you to easily create forms for your settings classes without having to manually create the form fields and use a declarative approach.
+
+## Retrieving form builders
+
+To get form builders containing 
+