docs: add migration guide for shadow variables (VariableListeners). (TimefoldAI#2053)

Solves part of TimefoldAI#2043 

This introduces a migration guide for migrating Variable Listeners to
Custom Shadow Variables (declarative).

- Introduced a new subheading in the "upgrade" docs. 
- Introduced a new page to describe this migration in detail. 
---------

Co-authored-by: Christopher Chianelli <christopher@timefold.ai>

Author: TomCools

docs/src/modules/ROOT/nav.adoc

@@ -35,4 +35,6 @@
 ** xref:upgrading-timefold-solver/upgrade-to-latest-version.adoc[leveloffset=+1]
 ** xref:upgrading-timefold-solver/upgrade-from-optaplanner.adoc[leveloffset=+1]
 ** xref:upgrading-timefold-solver/backwards-compatibility.adoc[leveloffset=+1]
+** Migration Guides
+*** xref:upgrading-timefold-solver/migration-guides/variable-listeners-to-custom-shadow-variables.adoc[leveloffset=+1]
 * xref:enterprise-edition/enterprise-edition.adoc[leveloffset=+1]

docs/src/modules/ROOT/pages/upgrading-timefold-solver/migration-guides/.migration-guides.adoc

@@ -0,0 +1,7 @@
+[#migrationGuides]
+= Migration Guides
+:doctype: book
+:sectnums:
+:icons: font
+
+include::variable-listeners-to-custom-shadow-variables.adoc[leveloffset=+1]