docs: customer-facing upgrade guide

There is no upgrade documentation under docs/user/, and docs/user/release-notes/ contains only an index.md.

A customer needs to be able to answer three things without experimenting on their cluster: where am I, where am I going, and what do I run. The rule should be short enough to repeat from memory — install the latest patch of each major version between yours and your target, in order, waiting for migrations between each.

Also needed: "minimum upgrade from" stated at the top of every release note, and a plain statement that you cannot roll back across a destructive migration except by backup and restore.