Register migrate/ as phantom in navigation.yml

Allows docs-content://migrate/* pages to pass link validation
without being in the primary nav tree. Required until migrate/
is wired into the top-level toc.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: alexandra5000

config/navigation.yml

@@ -11,6 +11,7 @@ phantoms:
   - toc: docs-content://release-notes/elasticsearch-clients
   - toc: docs-content://release-notes/apm-agents
   - toc: docs-content://
+  - toc: docs-content://migrate
   - toc: cloud://release-notes
 
 toc: