Restructure Migration Guide

Closes gh-12242

Author: jzheaux

docs/modules/ROOT/nav.adoc

@@ -2,7 +2,12 @@
 * xref:prerequisites.adoc[Prerequisites]
 * xref:community.adoc[Community]
 * xref:whats-new.adoc[What's New]
-* xref:migration.adoc[Migrating to 6.0]
+* xref:migration/index.adoc[Migrating to 6.0]
+** xref:migration/servlet/index.adoc[Servlet Migrations]
+*** xref:migration/servlet/session-management.adoc[Session Management]
+*** xref:migration/servlet/authentication.adoc[Authentication]
+*** xref:migration/servlet/authorization.adoc[Authorization]
+** xref:migration/reactive.adoc[Reactive Migrations]
 * xref:getting-spring-security.adoc[Getting Spring Security]
 * xref:features/index.adoc[Features]
 ** xref:features/authentication/index.adoc[Authentication]