Add note to 'Migrate to ILM' about existing indices

[kilfoyle]

Just a tiny PR to help folks find our page about applying an ILM policy to existing indices, and also reiterating the warning about not applying a policy that includes rollover.

Closes: #1567

[github-actions]

🔍 Preview links for changed docs

• manage-data/lifecycle/index-lifecycle-management.md

[shainaraskas]

read this and was a little confused until I clicked through. if you felt like it, you might be able to clarify this further by saying when you apply the ilm policy it's matching on the specific index name or whatever the underlying reason for the disconnection is, and uplevel the "apply to template" recommendation. if you think this will all be obvious to readers at this stage feel free to ignore.

[kilfoyle]

read this and was a little confused until I clicked through. if you felt like it, you might be able to clarify this further by saying when you apply the ilm policy it's matching on the specific index name or whatever the underlying reason for the disconnection is, and uplevel the "apply to template" recommendation. if you think this will all be obvious to readers at this stage feel free to ignore.

Thanks for the suggestion. In this case I think I'll leave the warning where it is as it's a bit of an edge case, and a different scenario that just setting up an index and managing it (that is, this is specific to folks who have a ton of indices set up and haven't named them in such a way to support rollover).