Update UniBackpack docs to the new structure

[michadasis]

Rewrites docs/unibackpack.md to follow the structure in Adding docs. The page mirrors the UniBackpack README, which was restructured the same
way in open-source-uom/UniBackpack#41.

Changes

• Description: what the app does, and how it works in four steps, including the apt list refresh and when an install can be cancelled.
• Installation:
  • For users: preinstalled on UniOS, pacman -S unibackpack from unios-repo, the PKGBUILD for other Arch distros, or building from source on Debian.
  • For developers: dependencies, then build, test and install steps.
• Project structure: a tree of the repository, sample universities.json and packages.json files, and the settings file the app writes.
• Guides: adding a department and adding a translation.
• Roadmap and recent changes, as the guide asks after a refactor. Both are kept in sync with the latest commits in Refactor/overhaul UniBackpack#41: Cancel button, apt list refresh, and the CI checks (warnings as errors, cppcheck, Doxygen).
• Contributors: maintainers plus the contributors graph.

Fixes to the old page

• Corrected the CMake version (3.16, not 3.10) and the Debian packages (pkexec, qt6-l10n-tools instead of policykit-1).
• Removed a stray "UniOS Logo" line in the middle of the install commands.
• The described file layout now matches the current code. The old page described the removed .txt package lists and called the icons per-department instead of per-university.

mkdocs.yml needs no change, since UniBackpack is already in the nav. The site builds with mkdocs build --strict without errors.

The message mentions UniBackpack#41, which still holds the matching README. Ideally the docs PR merges after #41, so the docs never describe code that isn't merged yet.

[TolisSth]

is this the one after the refactor or before ?

[michadasis]

After, it's mentioned in the description, it's supposed to be merged after we finalise the draft PR in unibackpack, both are drafts to signify that the code is not "solid" and can still be changed within the respective PRs