Add Plone 6.1 upgrade notes for plone.app.z3cform and mockup (#1750)

Thank you @petschki! This is awesome.


* Add upgrading notes for `plone.app.z3cform` and `mockup`

* Tidy up upgrade items for `z3c.form` and `plone.app.z3cform` and `mockup` new pattern `pat-contentbrowser`

---------

Co-authored-by: Steve Piercy <web@stevepiercy.com>

Author: petschki

docs/backend/upgrading/version-specific-migration/upgrade-to-61.md

@@ -64,16 +64,35 @@ To upgrade your plugin implementation to TinyMCE 7, see the [upgrade guides](htt
 
 ## `z3c.form` and `plone.app.z3cform`
 
-````{todo}
-This is a placeholder.
+[`plone.app.z3cform`](https://github.com/plone/plone.app.z3cform) is the form widget integration package for [`z3c.form`](https://github.com/zopefoundation/z3c.form) in Plone.
+This adds [Bootstrap 5](https://getbootstrap.com/) styling and mockup pattern options to all widgets.
 
--   Update deprecated imports
--   New widget templates
+In Plone 6.1 all Classic UI widget classes were moved to the module `plone.app.z3cform.widgets`.
+The previous paths are marked as deprecated and will be removed in Plone 7.
 
-```{seealso}
-https://github.com/plone/plone.app.z3cform/pull/181
-```
-````
+The `BaseWidget` for patterns is refactored to the new `z3c.form` extendable attributes introduced in version 5.1 and doesn't use LXML anymore.
+See https://github.com/zopefoundation/z3c.form/pull/116.
+If you have customizations in your base pattern widget class, see the new implementation at https://github.com/plone/plone.app.z3cform/blob/e9d1ebf478e663d2da259cb9435927f7ad1ddb92/plone/app/z3cform/widgets/base.py.
+
+`RelatedItemsWidget` is marked as deprecated.
+The implementation for selecting related items, internal links and images in TinyMCE, or internal paths for collection criteria is now done with the new `ContentBrowserWidget`.
+This introduces a new pattern `pat-contentbrowser` from mockup.
+See the next section for details.
+
+
+## `mockup` new pattern `pat-contentbrowser`
+
+A new content browsing pattern [`pat-contentbrowser`](https://plone.github.io/mockup/pat/contenbrowser/) for Classic UI is now available.
+
+This is a [Miller column browser](https://en.wikipedia.org/wiki/Miller_columns) implementation which replaces [`pat-relateditems`](https://plone.github.io/mockup/pat/relateditems/) seamlessly.
+All basic options from `pat-relateditems` are implemented and behave the same as before.
+
+Additionally `pat-contentbrowser` comes with some new features.
+
+-   Keyboard navigation.
+-   Multi-selection of items with {kbd}`Shift/Ctrl/CMD + click` combination.
+    This comes in handy for selecting multiple related items in one step.
+-   Uploading items to the current path.
 
 
 ## `plone.app.multilingual` is a core add-on