Add chapter to migration guide on how to migrate to Volto

Author: pbauer

docs/backend/upgrading/upgrade_to_volto.md

@@ -0,0 +1,79 @@
+---
+html_meta:
+  "description": "Upgrade to Volto"
+  "property=og:description": "Upgrade to Volto"
+  "property=og:title": "Upgrade to Volto"
+  "keywords": "Upgrading, Plone 6, Volto"
+---
+
+(backend-upgrade-to-volto-label)=
+
+# Upgrading to Volto
+
+To upgrade an existing site to run with Volto requires a couple of changes to your existing content.
+
+The form `/@@migrate_to_volto` allows you to run all these steps directly.
+After you upgraded your existing Database to Plone 6 there also is a message **You can prepare your site for Volto, the default frontend of Plone 6!** with a link to the form `localhost:8080/Plone/@@migrate_to_volto`.
+
+```{warning}
+Test all migrations thoroughly it before applying them on a production-environment!
+
+A migrated site will still work in Plone Classic but behave different.
+Editors can only work with the content effectively using Volto.
+```
+
+The required steps are:
+
+1. **Install the packages {py:mod}`plone.volto` and {py:mod}`plone.restapi`.**
+
+    {py:mod}`plone.restapi` is the RESTful hypermedia API for Plone that allows the Frontend Volto to communicate with the backend.
+    {py:mod}`plone.volto` configures Plone to work with Volto, the new default frontend for Plone 6.
+
+1. **Migrate Richtext-Fields to Volto blocks**
+
+    Volto has a new editor different from TinyMCE that is used in Classic Plone.
+    This step converts the html stored in RichText fields into text-blocks so you can edit it in Volto.
+    Images, Links and most kinds of html-formating are preserved.
+
+
+    For this you need to have `blocks-conversion-tool` running on an accessible url.
+    The easiest way to have that running on your machine is to use docker:
+
+    ```shell
+    docker run -it -p 5000:5000 plone/blocks-conversion-tool:latest
+    ```
+
+    For more options read https://github.com/plone/blocks-conversion-tool.
+
+1. **Pages, News Items and Events are made folderish**
+
+    That means these types can contain other content like Images or other Pages.
+    When you create a new site in Plone 6 this setting is also applied automatically.
+    But existing content remains non-folderish undtil this step is run.
+
+1. **Turn folders into folderish pages**
+
+    In Volto adding Folders is disabled by default.
+    Instead folderish pages are used to create folder-structures.
+    This step turn all folders into folderish pages.
+    If the folder showed a listing of the content a appropriate listing block will be added.
+    If the folder showed a default page that will be handled in the next step.
+    You can re-enable Folders by checking the box *Implicitly addable?* in ``/portal_types/Folder/manage_propertiesForm``.
+
+1. **Default Pages of Folders are merged with the Folderish Pages that replace the Folder**
+
+    Volto does not have a concept of default pages.
+    Instead folderish pages can show text and/or a listing of all the content inside that page.
+    This step takes the content of the default-page (e.g. text or the query of a collection) and adds that to the folderish page that replaced the folder.
+    Metadata (subjects, author, rights...) and relations are moved to the folderish page.
+
+
+1. **Collections are migrated to Pages with Listing Blocks**
+
+    In Volto adding Collections is disabled by default.
+    Instead folderish pages with listing blocks are used.
+    This step turns all collections into folderish pages.
+    The criteria of the collection are uses to configure a listing block in that page.
+
+
+It is recommended to use the default settings but you can choose to skip some of the migration-steps in the form.

docs/backend/upgrading/v60.md

@@ -55,6 +55,9 @@ The classic integrated frontend which is called "Classic UI" is still available
 If you choose to use Classic UI, then do not use the `plone.volto` package.
 ```
 
+To learn how to upgrade a existing site to run on Volto please read [Upgrading to Volto](backend-upgrade-to-volto-label)
+
+
 See [PLIP 2703](https://github.com/plone/Products.CMFPlone/issues/2703).