Nest two pages under 'Migrating to ILM' (elastic#1983)

Small fix to nest a couple of pages under the "Migrate to ILM" page.
This is because without that context, "Manage existing indices" could be
confused with the other two pages [Index management in
Kibana](https://www.elastic.co/docs/manage-data/lifecycle/index-lifecycle-management/index-management-in-kibana)
and [Index
management](https://www.elastic.co/docs/manage-data/data-store/index-basics#index-management).
I plan to combine those two latter pages as part of
elastic#1569

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src="https://github.com/user-attachments/assets/d1f0cb03-ea1c-4038-8411-ae27372c4871"
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Author: kilfoyle

manage-data/lifecycle/index-lifecycle-management/manage-existing-indices.md

@@ -9,7 +9,7 @@ products:
 
 # Manage existing indices [ilm-with-existing-indices]
 
-If you’ve been using Curator or some other mechanism to manage periodic indices, you have a couple options when migrating to {{ilm-init}}:
+If you’ve been using Curator or some other mechanism to manage periodic indices, you have a couple options when [migrating to {{ilm-init}}](./migrate-index-management.md):
 
 * Set up your index templates to use an {{ilm-init}} policy to manage your new indices. Once {{ilm-init}} is managing your current write index, you can apply an appropriate policy to your old indices.
 * Reindex into an {{ilm-init}}-managed index.

manage-data/toc.yml

@@ -127,9 +127,10 @@ toc:
           - file: lifecycle/index-lifecycle-management/restore-managed-data-stream-index.md
           - file: lifecycle/index-lifecycle-management/tutorial-customize-built-in-policies.md
           - file: lifecycle/index-lifecycle-management/index-management-in-kibana.md
-          - file: lifecycle/index-lifecycle-management/manage-existing-indices.md
           - file: lifecycle/index-lifecycle-management/migrate-index-management.md
-          - file: lifecycle/index-lifecycle-management/migrate-index-allocation-filters-to-node-roles.md
+            children:
+              - file: lifecycle/index-lifecycle-management/manage-existing-indices.md
+              - file: lifecycle/index-lifecycle-management/migrate-index-allocation-filters-to-node-roles.md
       - file: lifecycle/data-stream.md
         children:
           - file: lifecycle/data-stream/tutorial-create-data-stream-with-lifecycle.md