add form intro, chapter general forms and edit/add forms

Author: MrTango

docs/classic-ui/forms.md

@@ -14,4 +14,90 @@ myst:
 ```{todo}
 Describe how create forms with Plone's default form framework z3c.form.
 Fields, Widgets, Vocabularies aso are descripted in detail in there own chapter and will be referenced frome examples here.
-```
+```
+
+Plone uses the [z3c.form](http://pythonhosted.org/z3c.form) library to build its forms, via the [plone.z3cform](http://pypi.python.org/pypi/plone.z3cform) integration package.
+
+For an easy way to structure a form and define its widgets and fields, Plone relies on [plone.autoform](http://pypi.python.org/pypi/plone.autoform), in particular its AutoExtensibleForm base class.
+Which is responsible for processing form hints and setting up [z3c.form](http://pythonhosted.org/z3c.form) widgets and groups (fieldsets).
+
+A form, therefore, is simply a view that uses these libraries, although [plone.autoform](http://pypi.python.org/pypi/plone.autoform) provides some helpful base classes that make it easier to construct forms based on the form schema or {ref}`Dexterity behaviors <backend-behaviors-label>`.
+
+
+(classic-ui-forms-general-forms-label)=
+## General forms
+
+The plonecli provides you with an option to add a form to your Plone package.
+
+
+```shell
+cd collective.awesomeaddon
+plonecli add form
+```
+
+After using the `plonecli` to add a form, we should have a new sub folder `forms` in our package.
+Here we will find a `configure.zcml` containing the registration of the form,
+
+```xml
+<browser:page
+  name="my-form"
+  for="*"
+  class=".my_form.MyForm"
+  permission="cmf.ManagePortal"
+  layer="p6.theme5.interfaces.IP6Theme5Layer"
+  />
+```
+
+and a Python file with the code.
+
+```python
+from plone import schema
+from plone.autoform.form import AutoExtensibleForm
+from z3c.form import button, form
+from zope.interface import Interface
+
+
+class IMyForm(Interface):
+    """ Schema Interface for IMyForm
+        Define your form fields here.
+    """
+    name = schema.TextLine(
+        title=u"Your name",
+    )
+
+
+class MyForm(AutoExtensibleForm, form.EditForm):
+    schema = IMyForm
+    ignoreContext = True
+
+    label = u"What's your name?"
+    description = u"Simple, sample form"
+
+    @button.buttonAndHandler(u'Ok')
+    def handleApply(self, action):
+        data, errors = self.extractData()
+        if errors:
+            self.status = self.formErrorsMessage
+            return
+
+        # Do something with valid data here
+
+        changes = self.applyChanges(data)
+        # Set status on this form page
+        # (this status message is not bind to the session and does not go thru redirects)
+        if changes:
+            self.status = "Settings saved"
+
+    @button.buttonAndHandler(u"Cancel")
+    def handleCancel(self, action):
+        """User canceled. Redirect back to the front page.
+        """
+
+```
+
+
+(classic-ui-forms-dexterity-add-edit-forms-label)=
+## Dexterity Add- and Edit forms
+
+Dexterity content types are coming with default add and edit forms.
+You can build custom add and edit forms if you need.