* Add docs/just.md as dedicated documentation of "just" commands

This is partially based on fb60ba67f6 (announcement of adoption of "just" program). It also refers descriptions on installing.md.

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

* Create a table for examples

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

* Fix entries on the table

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

* Move the anchor link to "agru"

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

* Edit docs/faq.md: add an entry for the just

It is based on the existing explanation of the just on docs/maintenance-upgrading-services.md.

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

* Add links to docs/just.md

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

* Update docs/just.md: add a common note

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

* Clarify "What is just" section on FAQ

* Update just.md

* Mention install-service

---------

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>
Co-authored-by: Suguru Hirahara <acioustick@noreply.codeberg.org>
Co-authored-by: Slavi Pantaleev <slavi@devture.com>