docs: add link to upgrade from OptaPlanner

Author: triceo

docs/src/modules/ROOT/nav.adoc

@@ -34,6 +34,7 @@
 * Upgrading Timefold Solver
 ** xref:upgrading-timefold-solver/overview.adoc[leveloffset=+1]
 ** xref:upgrading-timefold-solver/upgrade-from-v1.adoc[leveloffset=+1]
+** https://docs.timefold.ai/timefold-solver/1.x/upgrading-timefold-solver/upgrade-from-optaplanner[Upgrading from OptaPlanner][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]

docs/src/modules/ROOT/pages/upgrading-timefold-solver/upgrade-from-v1.adoc

@@ -1,7 +1,7 @@
 :page-aliases: upgrading-timefold-solver/upgrade-to-latest-version.adoc
 
 [#upgradeFrom1to2]
-= Upgrade from Timefold Solver 1.x to 2.0.0
+= Upgrade from Timefold Solver 1.x to 2.x
 :doctype: book
 :sectnums:
 :icons: font