Set the boot splash to blob_boot.png during install

docs/install.md

@@ -15,6 +15,7 @@ with `--check`.
 | `install/steps/packages.sh` | repo and AUR packages, and the yay bootstrap |
 | `install/steps/config.sh` | the `BLOB_PATH` symlink, everything under `~/.config`, and the login shell profile |
 | `install/steps/services.sh` | system services, systemd drop-ins, user units |
+| `install/steps/boot.sh` | the Plymouth boot splash |
 | `install/steps/theme.sh` | the wallpaper link, the first theme, the first background |
 | `install/steps/login.sh` | SDDM, the session entry, and the first launch |
 | `install/steps/reload.sh` | reloading systemd, the user services, and a running session |
@@ -78,7 +79,11 @@ with `--check`.
    hold up the login screen, and `plymouth-start.service` and
    `plymouth-quit-wait.service` are masked because an unconfigured plymouth
    holds the console and leaves the machine with neither a greeter nor a TTY.
-   `blob boot` unmasks them when it sets the splash up. See
+   The installer then applies `branding/blob_boot.png` as the boot splash
+   through `blob boot`, which installs the theme, adds the mkinitcpio hook,
+   unmasks the plymouth units and rebuilds the initramfs. It is skipped when
+   the blob theme is already the default with the same image, and once it is
+   in place the plymouth units are no longer masked on later runs. See
    [packages.md](packages.md#the-boot-splash).
 9. **The login screen** is SDDM. It has to be installed first: if
    `sddm.service` is not there, the installer says whether the package is