Point Tails Upgrader to /doc/upgrade

It was renamed in d3ced622fa to make our documentation index easier to read.

The redirection in #17146 (closed) will take care of older Tails versions so it’s not super urgent.

Feature Branch: iuk.git:feature/17151-rename-doc-url

Original created by @sajolida on 17151 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information