Upgrade docs - introduce better the archived settings and archive functionality of old indices

[eedugon]

These docs need some work:

Archive functionality
archived settings

Those docs are applicable to all deployment types, so they shouldn't be under self-managed upgrade in the doc.

Also the topics need to be introduced a bit better into the upgrade docs.

For example archived settings should be checked before a major upgrade, and that's not in the prepare-upgrade steps doc. Also the doc says:

We recommend you remove any archived settings after upgrading

So we need to think if it makes sense to introduce that somehow in the flow, as currently there are no post-upgrade steps.

• For the archive functionality it would be great to clarify how exactly to use it, if the user needs to do anything. As reading the doc it's not clear at all. Also the archive functionality through snapshots can be felt in the doc as the only way, where it's not. All that need to be refined and clarified.

As mentioned in another issue about the archive functionality doc, the value it currently provides adds more questions than answers :)

My plan is to do some tests to clarify its use or contact devs to get some insights.