docs: add 2.5->2.6 upgrade notes to table of contents (argoproj#12319)

Signed-off-by: Michael Crenshaw <350466+crenshaw-dev@users.noreply.github.com>

mkdocs.yml

@@ -113,6 +113,7 @@ nav:
     - operator-manual/server-commands/additional-configuration-method.md
   - Upgrading:
     - operator-manual/upgrading/overview.md
+    - operator-manual/upgrading/2.5-2.6.md
     - operator-manual/upgrading/2.4-2.5.md
     - operator-manual/upgrading/2.3-2.4.md
     - operator-manual/upgrading/2.2-2.3.md