4.x: include SE upgrade guide in docs navbar (#8795)

* Add upgrade_4x to sitegen.yaml for MP. Remove updgrade 2.x/3.x for SE
* Fix flavor typo in upgrad_4x SE doc

docs/src/main/asciidoc/se/guides/upgrade_4x.adoc

@@ -25,7 +25,7 @@ include::{rootdir}/includes/attributes.adoc[]
 
 In Helidon 4.x we have made some major changes to Helidon. Reactive engine has been removed.
 APIS and implementations are rewritten in "blocking" paradigm.
-This guide will help you upgrade a Helidon MP 3.x application to 4.x.
+This guide will help you upgrade a Helidon SE 3.x application to 4.x.
 
 
 == Java 21 Runtime

docs/src/main/asciidoc/sitegen.yaml

@@ -91,6 +91,7 @@ backend:
                   - "security-oidc.adoc"
                   - "tracing.adoc"
                   - "mp-tutorial.adoc"
+                  - "upgrade_4x.adoc"
                   - "maven-build.adoc"
                   - "gradle-build.adoc"
                   - "graalnative.adoc"
@@ -318,8 +319,6 @@ backend:
                   - "metrics.adoc"
                   - "security-oidc.adoc"
                   - "tracing.adoc"
-                  - "upgrade.adoc"
-                  - "upgrade_3x.adoc"
                   - "upgrade_4x.adoc"
                   - "maven-build.adoc"
                   - "gradle-build.adoc"