Restructure upgrade section for instance types

What does this MR do and why?

While getting gitlab-org/gitlab!209230 reviewed, it became clear that the multi-node topics shouldn't be places as sub-pages of Linux package upgrades.

While making the change, I've refined and reordered the items.

• Before: https://docs.gitlab.com/update/upgrade/
• After: https://docs.gitlab.com/review-mr-1444/update/upgrade/

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