Add docs/just.md (#3811)

* 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

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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>

docs/prerequisites.md

@@ -14,7 +14,7 @@ We will be using `example.com` as the domain in the following instruction. Pleas
 
 - [`git`](https://git-scm.com/) as the recommended way to download the playbook. `git` may also be required on the server if you will be [self-building](self-building.md) components.
 
-- [`just`](https://github.com/casey/just) for running `just roles`, `just update`, etc. (see [`justfile`](../justfile)), although you can also run these commands manually
+- [`just`](https://github.com/casey/just) for running `just roles`, `just update`, etc. (see [`justfile`](../justfile)), although you can also run these commands manually. Take at look at this documentation for more information: [Running `just` commands](just.md).
 
 - Strong password (random strings) generator. The playbook often requires you to create a strong password and use it for settings on `vars.yml`, components, etc. As any tools should be fine, this playbook has adopted [`pwgen`](https://linux.die.net/man/1/pwgen) (running `pwgen -s 64 1`). [Password Tech](https://pwgen-win.sourceforge.io/), formerly known as "PWGen for Windows", is available as free and open source password generator for Windows. Generally, using a random generator available on the internet is not recommended.