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>

docs/just.md

@@ -0,0 +1,35 @@
+# Running `just` commands
+
+We have previously used [make](https://www.gnu.org/software/make/) for easily running some playbook commands (e.g. `make roles` which triggers [`ansible-galaxy`](https://docs.ansible.com/ansible/latest/cli/ansible-galaxy.html)). Our [`Makefile`](../Makefile) is still around, and you can still run these commands.
+
+In addition, we have added support for running commands via [`just`](https://github.com/casey/just) - a more modern command-runner alternative to `make`. It can be used to invoke `ansible-playbook` commands with less typing.
+
+The `just` utility executes shortcut commands (called as "recipes"), which ultimately run `ansible-galaxy`. The targets of the recipes are defined in [`justfile`](../justfile). It defines commands, most of which are not available on our `Makefile`.
+
+For some recipes such as `just update`, our `justfile` recommends to install [agru](https://github.com/etkecc/agru#where-to-get) to speed up the process.
+
+Here are some examples of shortcuts:
+
+- `just roles` - trigger `make roles` to install the necessary Ansible roles pinned in [`requirements.yml`](../requirements.yml)
+- `just update` - run `git pull` to update the playbook and `just roles`
+- `just install-all` - run `ansible-playbook -i inventory/hosts setup.yml --tags=install-all,ensure-matrix-users-created,start`
+    - `just install-all --ask-vault-pass` - commands also support additional arguments (`--ask-vault-pass` will be appended to the above installation command)
+- `just setup-all` - run `ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,ensure-matrix-users-created,start`
+- `just run-tags install-mautrix-slack,start` - run specific playbook tags
+- `just start-all` - (re-)starts all services
+- `just stop-group postgres` - stop only the Postgres service
+- `just register-user john secret-password yes` - registers a `john` user with the `secret-password` password and admin access (admin = `yes`)
+
+While [our documentation on prerequisites](prerequisites.md) lists `just` as one of the requirements for installation, using `just` is optional. If you find it difficult to install it, do not find it useful, or want to prefer raw `ansible-playbook` commands for some reason, feel free to run all commands manually. For example, you can run `ansible-galaxy` directly to install the Ansible roles: `rm -rf roles/galaxy; ansible-galaxy install -r requirements.yml -p roles/galaxy/ --force`.
+
+## Difference between playbook tags and shortcuts
+
+It is worth noting that `just` "recipes" are different from [playbook tags](playbook-tags.md) themselves. The recipes are shortcuts of commands defined in `justfile` and can be executed by the `just` program only, while the playbook tags are available for the raw `ansible-playbook` commands as well. Please be careful not to confuse them.
+
+For example, these two commands are different:
+- `just install-all`
+- `ansible-playbook -i inventory/hosts setup.yml --tags=install-all`
+
+The just recipe runs `ensure-matrix-users-created` and `start` tags after `install-all`, while the latter runs only `install-all` tag. The correct shortcut of the latter is `just run-tags install-all`.
+
+Such kind of difference sometimes matters. For example, when you install a Matrix server, into which you will import old data (see [here](installing.md#installing-a-server-into-which-youll-import-old-data)), you are not supposed to run `just install-all` or `just setup-all`, because these commands start services soon after installing components, and will affect your data which will be manually imported later.

docs/playbook-tags.md

@@ -20,4 +20,6 @@ Here are some playbook tags that you should be familiar with:
 
 - `ensure-matrix-users-created` - a special tag which ensures that all special users needed by the playbook (for bots, etc.) are created
 
-`setup-*` tags and `install-*` tags **do not start services** automatically, because you may wish to do things before starting services, such as importing a database dump, restoring data from another server, etc.
+**Notes**:
+- `setup-*` tags and `install-*` tags **do not start services** automatically, because you may wish to do things before starting services, such as importing a database dump, restoring data from another server, etc.
+- Please be careful not to confuse the playbook tags with the `just` shortcut commands ("recipes"). For details on `just` commands, see: [Running `just` commands](just.md)