Also: move descriptions about difference between the playbook tags (setup-all and install-all) and about the just "recipe" from installing.md to maintenance-upgrading-services.md

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

improves readability

* rewrite `just update` command to provide a one-line command to update everything

* update prefix

* uncomment update-self

* Revert requirements.yml updates not belonging to this PR

* Justfile and documentation updates to make things clearer

---------

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

We keep the Makefile for now, but don't mention it to new
users anymore.