feat: Restructure upgrade docs

[mauromorales]

While working on the kairos high-level docs, I want to introduce some changes in the documentation. Here's one:

The upgrade docs are grouped this way atm. IMO we should remove the installation of the Kairos Operator and of the deprecated SUC. This section should only talk about how to upgrade manually, from kubernetes, trusted boot, etc. the kairos operator and deprecated suc should go on Advanced, Reference or a new section.

[jimmykarily]

I agree that the deprecated page doesn't belong in the menu. The "From Kubernetes" page links to the "Kairos Operator" one so I also agree that the "Kairos Operator" can be moved to "Reference".