Add upgrade tool documentation to migration guides

Add references to the rector-based upgrade tool in the migration guides
index page and in each minor version migration guide (5.1, 5.2, 5.3).

This improves discoverability of the upgrade tool which has rulesets
for each minor version to help automate code changes.

Fixes #8056

Author: dereuromark

en/appendices/5-1-migration-guide.rst

@@ -5,6 +5,15 @@ The 5.1.0 release is a backwards compatible with 5.0. It adds new functionality
 and introduces new deprecations. Any functionality deprecated in 5.x will be
 removed in 6.0.0.
 
+Upgrade Tool
+============
+
+The :doc:`upgrade tool </appendices/migration-guides>` provides rector rules for
+automating some of the migration work. Run rector before updating your
+``composer.json`` dependencies::
+
+    bin/cake upgrade rector --rules cakephp51 <path/to/app/src>
+
 Behavior Changes
 ================
 

en/appendices/5-2-migration-guide.rst

@@ -5,6 +5,15 @@ The 5.2.0 release is a backwards compatible with 5.0. It adds new functionality
 and introduces new deprecations. Any functionality deprecated in 5.x will be
 removed in 6.0.0.
 
+Upgrade Tool
+============
+
+The :doc:`upgrade tool </appendices/migration-guides>` provides rector rules for
+automating some of the migration work. Run rector before updating your
+``composer.json`` dependencies::
+
+    bin/cake upgrade rector --rules cakephp52 <path/to/app/src>
+
 Behavior Changes
 ================
 

en/appendices/5-3-migration-guide.rst

@@ -5,6 +5,15 @@ The 5.3.0 release is a backwards compatible with 5.0. It adds new functionality
 and introduces new deprecations. Any functionality deprecated in 5.x will be
 removed in 6.0.0.
 
+Upgrade Tool
+============
+
+The :doc:`upgrade tool </appendices/migration-guides>` provides rector rules for
+automating some of the migration work. Run rector before updating your
+``composer.json`` dependencies::
+
+    bin/cake upgrade rector --rules cakephp53 <path/to/app/src>
+
 Upgrade to PHP 8.2
 ==================
 

en/appendices/migration-guides.rst

@@ -4,6 +4,28 @@ Migration Guides
 Migration guides contain information regarding the new features introduced in
 each version and the migration path between 5.x minor releases.
 
+Upgrade Tool
+============
+
+CakePHP provides an `upgrade tool <https://github.com/cakephp/upgrade>`__ that
+automates many code changes using `Rector <https://getrector.com/>`__. The tool
+has rulesets for each minor version to help automate tedious code changes like
+method renames and signature updates.
+
+To use the upgrade tool::
+
+    # Install the upgrade tool
+    git clone https://github.com/cakephp/upgrade
+    cd upgrade
+    git checkout 5.x
+    composer install --no-dev
+
+    # Run rector with the desired ruleset
+    bin/cake upgrade rector --rules cakephp51 <path/to/app/src>
+
+Run rector before updating your ``composer.json`` dependencies
+to ensure the tool can resolve class names correctly.
+
 .. toctree::
     :maxdepth: 1