Add docs for ILM feature

[ruflin]

This is a follow up for #7963

[andrewkroh]

I left a few suggestions. After reading that I feel like I know how to use ILM so I think they are good.

[ruflin]

@andrewkroh Thanks for the early review. Will also need to fix some urls.

[ruflin]

Assigned @dedemorton on this PR as she will take it over.

[dedemorton]

I still need to rebase, but the content here is ready for a review. I've moved the info to its own topic. It needs more work, but would like to get it merged before/around the time 6.6 hits the fan.

[ruflin]

One minor comment. Overall LGTM.

[ruflin]

I added the needs_backport label as this will have to go into 6.6 and 6.x too.

[dedemorton]

I've rebased and cleaned up some of the cross reference text to be consistent with how other writers are handling cross references. I think this is ready to merge.

I still need to update the security docs, but I'll do that in a separate PR.

@ruflin What's required wrt privileges? #9470 suggests:

{ "cluster": ["manage_index_templates","monitor"],
"indices": [ { "names": [ "beat-" ], "privileges": ["write","delete","create_index"] } ] }

If this is correct, I'll update the security docs next week. Thanks!

[dedemorton]

@bmorelli25 If you want to pick up the ILM docs for 6.6, you'll need to grab this after it's merged and add an entry to the topic that pulls in the config topics. I'm assuming ILM will work for APM indices, no? Sorry this is landing so late.

[bmorelli25]

@dede I don't think ILM support is coming to APM until after 7.0, but I'll need to double check.

[ruflin]

@dedemorton LGTM. Can't approve as it's my own PR. Feel free to merge.

For the privileges: Can you check with @jakelandis to confirm this?