Add the warning message for wiki migration (#52547)

Co-authored-by: Ben Ahmady <32935794+subatoi@users.noreply.github.com>
Co-authored-by: Sophie <29382425+sophietheking@users.noreply.github.com>

content/migrations/using-github-enterprise-importer/migrating-between-github-products/about-migrations-between-github-products.md

@@ -22,6 +22,17 @@ The data that {% data variables.product.prodname_importer_proper_name %} migrate
 
 ## Data that is migrated from {% data variables.product.prodname_ghe_server %}
 
+> [!WARNING]
+> The Wikis migration is currently unavailable.
+> As a workaround, you can migrate manually:
+>
+> ```shell copy
+> git clone --mirror OLD-REPOSITORY-URL
+> cd OLD-REPOSITORY-NAME
+> git remote add new-origin NEW-REPOSITORY-URL
+> git push new-origin --mirror
+> ```
+
 To migrate from {% data variables.product.prodname_ghe_server %} (GHES), you must have GHES version 3.4.1 or higher. The data that is migrated depends on the version you're using.
 
 Item | GHES 3.4.1+ | GHES 3.5.0+ |