incremental upgrades: user documentation

Users must be explained how to download and install an Incremental Upgrade Kit.

Missing pages:

• doc/upgrade/error/check: the user is pointed to this page when the upgrade-description file cannot be downloaded or verified
• doc/upgrade/error/download: the user is pointed to this page when a target file file cannot be downloaded or verified
• doc/upgrade/error/install: the user is pointed to this page when the installation of a downloaded IUK failed. This page should be about what to do in this case. My Tails may be left in a broken state, as long as I am told what broke, and explained how to repair it (possibly by re-installing from scratch, or by performing a full update).

Feature Branch: feature/incremental-upgrades-integration

Parent Task: #6014 (closed)

Related issues

• Blocked by #6451 (closed)
• Blocks #5754 (closed)

Original created by @tails on 5906 (Redmine)