flash: make configuration more explicit

Injecting nixos configuration and potentially overriding settings a user
made and can cause surprises.
In most cases, users want to just make these option part of their NixOS
configuration and by having the rest in the command line
we make it more explicit what other configuration is being applied.

docs/site/getting-started/installer.md

@@ -42,13 +42,22 @@ sudo umount /dev/sdb1
 === "**Linux OS**"
     ### Step 2. Flash Custom Installer
 
-    Using clan flash enables the inclusion of ssh public keys and disables ssh password authentication.
-    It also includes the language and keymap currently used into the installer image.
+    Using clan flash enables the inclusion of ssh public keys.
+    It also allows to set language and keymap currently in the installer image.
 
     ```bash
-    clan flash --flake git+https://git.clan.lol/clan/clan-core flash-installer --disk main /dev/sd<X>
+    clan flash --flake git+https://git.clan.lol/clan/clan-core \
+      --ssh-pubkey $HOME/.ssh/id_ed25519.pub \
+      --keymap en \
+      --language en \
+      --disk main /dev/sd<X> \
+      flash-installer
     ```
 
+   The `--ssh-pubkey`, `--language` and `--keymap` are optional.
+   Replace `$HOME/.ssh/id_ed25519.pub` with a path to your SSH public key.
+   If you do not have an ssh key yet, you can generate one with `ssh-keygen -t ed25519` command.
+
     !!! Danger "Specifying the wrong device can lead to unrecoverable data loss."
 
         The `clan flash` utility will erase the disk. Make sure to specify the correct device