Add Viewlets section to 6.0 backend upgrade guide.

See also plone/Products.CMFPlone#3911

Author: mauritsvanrees

docs/backend/upgrading/version-specific-migration/upgrade-to-60.md

@@ -602,3 +602,40 @@ Please make sure you write valid JSON for the `template` option.
 ```{seealso}
 See also the [TinyMCE 4 to 5 upgrade guide](https://www.tiny.cloud/docs/migration-from-4x/).
 ```
+
+## Viewlets
+
+Plone 6.0 renames various viewlets or moves them to a different viewlet manager.
+The reason for renaming, is that some viewlet names contained the name of a viewlet manager.
+This didn't always match the name of their actual viewlet manager, especially after moving them.
+Plone 6.0 removes such references from the viewlet names, to avoid confusion.
+
+- Plone 6.0 removes the `plone.documentactions (IDocumentActions)`  viewlet manager. In Plone 5.2 it was already empty.
+- Plone 6.0 adds the `plone.belowcontentdescription (IBelowContentDescription)` viewlet manager. By default this has no viewlets.
+- Plone 6.0 removes the `plone.header` viewlet from `plone.portaltop` manager, making it empty.
+- Plone 6.0 renames the `plone.abovecontenttitle.documentactions` viewlet to `plone.documentactions` and moves it from manager `plone.belowcontentbody` to `plone.belowcontent`.
+- Plone 6.0 renames the `plone.abovecontenttitle.socialtags` viewlet to `plone.socialtags`. It remains in manager `plone.abovecontenttitle`.
+- Plone 6.0 renames the `plone.belowcontentbody.relateditems` viewlet to `plone.relateditems`. It remains in manager `plone.belowcontentbody`.
+- Plone 6.0 removes the `plone.manage_portlets_fallback` viewlet from the `plone.belowcontent` manager.
+- Plone 6.0 renames the `plone.belowcontenttitle.documentbyline` viewlet to `plone.documentbyline` . It remains in manager `plone.belowcontenttitle`.
+- Plone 6.0 renames the `plone.belowcontenttitle.keywords` viewlet to `plone.keywords` and moves it from manager `plone.belowcontent` to `plone.belowcontentbody`.
+- Plone 6.0 adds the `plone.rights` viewlet in manager `plone.belowcontentbody`.
+- Plone 6.0 moves the `plone.footer` viewlet from `plone.app.layout/viewlets` to `plone.app.portlets`. The viewlet remains in manager `plone.portalfooter`. It renders the portlets from the `plone.footerportlets` portlet manager.
+
+This is the same information in table form.
+You can toggle navigation to make more of the table visible.
+
+:::{table} Viewlet changes from 5.2 to 6.0
+
+| 5.2 viewlet name | 5.2 viewlet manager | 6.0 viewlet name | 6.0 viewlet manager |
+| ---------------- | ------------------- | ---------------- | ------------------- |
+|| `plone.documentactions` | | |
+|||| `plone.belowcontentdescription` |
+| `plone.header` | `plone.portaltop` | | `plone.portaltop` |
+| `plone.abovecontenttitle.documentactions` | `plone.belowcontentbody` | `plone.documentactions` | `plone.belowcontent` |
+| `plone.abovecontenttitle.socialtags` | `plone.abovecontenttitle` | `plone.socialtags` | `plone.abovecontenttitle` |
+| `plone.belowcontentbody.relateditems` | `plone.belowcontentbody` | `plone.relateditems` | `plone.belowcontentbody` |
+| `plone.manage_portlets_fallback` | `plone.belowcontent` | | `plone.belowcontent` |
+| `plone.belowcontenttitle.documentbyline` | `plone.belowcontenttitle` | `plone.documentbyline` | `plone.belowcontenttitle` |
+| `plone.belowcontenttitle.keywords` | `plone.belowcontent` | `plone.keywords` | `plone.belowcontentbody` |
+| | `plone.belowcontentbody` | `plone.rights` | `plone.belowcontentbody` |