fix errors and include old addon migration infos

Author: pbauer

docs/backend/upgrading/intro.md

@@ -81,7 +81,7 @@ It just fixed a bug.
 A post on the Community forum, [Rules for Plone 6 development during the beta stage](https://community.plone.org/t/rules-for-plone-6-development-during-the-beta-stage/15432), discusses alpha and beta versioning.
 ```
 
-In addition to the general procedure there are {doc}`version-specific migration guides </manage/upgrading/version_specific_migration/index>`.
+In addition to the general procedure there are {ref}`version-specific-upgrade-guide-label`.
 
 (introduction-version-specific-upgrade-guides-label)=
 

docs/backend/upgrading/troubleshooting.md

@@ -29,8 +29,8 @@ bin/instance fg
 
 You may be able to find more information on the error messages in:
 
-- the {doc}`Version-specific migration tips </backend/upgrading/version_specific_migration/index>` for your version of Plone
-- the {doc}`Error References </appendices/error-reference>`
+- the {doc}`Version-specific migration tips </backend/upgrading/version-specific-migration/index>` for your version of Plone
+- the https://docs.plone.org/appendices/error-reference.html
 
 ## Test without customizations
 
@@ -63,7 +63,7 @@ If the problem does not occur in a fresh site, the cause of your problem is most
 
 ## Make the problem reproducible
 
-Before you go out and {doc}`ask for help </askforhelp>`, you should be able to describe your problem in such a way that others can reproduce it in their environment.
+Before you go out and [ask for help](https://docs.plone.org/askforhelp.html), you should be able to describe your problem in such a way that others can reproduce it in their environment.
 
 Reduce the problem to the smallest possible domain.
 Eliminate products and customizations that are not part of the problem.
@@ -72,7 +72,7 @@ The more complex your story is, the more likely that it is unique to your situat
 
 ## Ask for help
 
-{doc}`Ask for help </askforhelp>` in the [Plone support channels](https://plone.org/support). Be sure to:
+[Ask for help](https://docs.plone.org/askforhelp.html) in the [Plone support channels](https://plone.org/support). Be sure to:
 
 - Provide relevant source code for your customizations that are part of the problem.
 - Describe the exact configuration, software versions, migration history, error messages and so on.

docs/backend/upgrading/version-specific-migration/images/content-type-icons.png

@@ -1,3 +1,14 @@
+---
+myst:
+  html_meta:
+    "description": "Version-specific upgrade guide"
+    "property=og:description": "Version-specific upgrade guide"
+    "property=og:title": "Version-specific upgrade guide"
+    "keywords": "Upgrading"
+---
+
+(version-specific-upgrade-guide-label)=
+
 # Version-specific migration procedures and tips
 
 ```{admonition} Description
@@ -9,8 +20,10 @@ If your migration does not involve a version pair specified here, then you may f
 :maxdepth: 2
 
 p4x-to-p5x-upgrade
+upgrade-addons-to-50
 plone5-minor-upgrade
 upgrade-to-51
+upgrade-addons-to-51
 upgrade-to-52
 upgrade-to-python3
 upgrade-zodb-to-python3

docs/backend/upgrading/version-specific-migration/images/plip_hidpi_high_resolution.png

@@ -25,15 +25,17 @@ This document discusses these differences.
 It also informs administrators and developers of how to migrate their existing Plone 6 site with Classic UI to instead become compatible with Volto for its frontend.
 
 ```{important}
-As a pre-requisite, your Plone site must be [upgraded to Plone 6](v60) before migrating to Volto for the frontend.
+
+As a pre-requisite, your Plone site must be {ref}`upgraded to Plone 6<backend-upgrade-plone-v60-label>` before migrating to Volto for the frontend.
+
 ```
 
 Plone provides a form `/@@migrate_to_volto` that allows you to run all the required changes to your existing site to make it compatible with Volto.
 
 You can access this form in the browser when you are logged in as an administrator.
 Open `http://localhost:8080/Plone/@@migrate_to_volto`, where `localhost` is your hostname, `8080` is the port on which Plone runs, and `Plone` is the name of the Plone instance.
 
-Additionally, after upgrading an existing site to Plone 6 (see {doc}`v60`), a message will appear, **You can prepare your site for Volto, the default frontend of Plone 6!**, with a link to that form.
+Additionally, after upgrading an existing site to Plone 6 (see {ref}`backend-upgrade-plone-v60-label`), a message will appear, **You can prepare your site for Volto, the default frontend of Plone 6!**, with a link to that form.
 
 ```{warning}
 Test all migrations thoroughly before applying them on a production environment!

docs/backend/upgrading/version-specific-migration/images/plip_hidpi_low_resolution.png

@@ -5,7 +5,7 @@ Instructions and tips for upgrading to a newer Plone version.
 ```
 
 ```{note}
-If you want to upgrade add-ons to Plone 5, also see {doc}`/develop/addons/upgrade-to-50`
+If you want to upgrade add-ons to Plone 5, also see {ref}`upgrading-addons-to-50-label`
 ```
 
 ## General Information
@@ -410,7 +410,7 @@ To have an icon for your control panel you must make sure that a css rule exists
 
 An example might be
 
-```css
+```
 .icon-controlpanel-MyPackage:before { content: ‘\e844’; }
 ```
 

docs/backend/upgrading/version-specific-migration/images/site-control-panel-icon-thumb-settings.png

@@ -6,9 +6,9 @@ Steps for minor upgrades within the Plone 5 Major Release.
 
 ```{warning}
 Before performing any Plone upgrade, you should always have a complete backup of your site.
-See the {doc}`Preparations </manage/upgrading/preparations>` section of this manual for more details.
+See the {ref}`upgrade-preparations-label` section of this manual for more details.
 
-In addition, you should check the {doc}`Version-specific migration tips </manage/upgrading/version_specific_migration/index>`
+In addition, you should check the {ref}`version-specific-upgrade-guide-label`
 section of this manual for any notes that may apply to the specific version upgrade you're about to perform.
 ```